I am within striking distance of finishing my Krag 35/40 sporter. Looking at starting loads for 308 W with heavy bullets sometimes they are right at 40K CUP and the same powder in a Krag at 40K CUP (was PSI)  shows the same charges with Varget. My go to powder has been Varget for some time and Hodgdon shows;
46 grains at 42,800 CUP load for the 358W with a 220 bullet
45 Grains at 40700 CUP load for the 358W with a 225 bullet
34 grains at 27800 CUP load for 35Rem with a 220 bullet
38 grains at 33300 CUP load for 35 Rem with a 220 Bullet
Hodgdon does not seem to be worried about detonation with low pressure loads in the 35Rem with Varget.
Lyman's reloading book had very similar data for 30/40 308W and 358W. I will have to borrow my son's Lyman book to verify.
So the question is does 40 grains as a starting load and 45 grains as a maximum load sound reasonable for Varget in a 35/40? One advantage of using loads like this in Varget, there will be no chance of getting an accidental double charge.

Mr. Steinmayer's advice is correct.  The action is a Krag, not something chambered for .308 or .358.  40.0 grains of IMR 4350 pushes a 220 grain jacketed .30 cal bullet at something approaching the velocity of the original Krag cartridge. 

If you use the same 40 grain load in a Krag case opened to .35 caliber, and you are pushing a 220 grain bullet, the peak pressure will be about the same, but pressures will drop quicker because the gas is escaping from a tube that has a bore cavity of greater capacity.

If you are necking the 30/40 case to 35 caliber you are making a 1/10 in short 35 Winchester case.  Not sure how much 35 Winchester data is still around (let alone pressure tested) but you can use the starting load data as a guide.  The 35 Win was used in the 1895 Winchester lever and had a working pressure of about 45K psi vice the 40K psi for a Krag.  The case capacity comparison given for 358 Win and the 30/40 makes them seem very similar, but the data I have seen is based on the case filled to the mouth, meaning the actual powder space in the Krag is a lot less than in the 358 and probably a lot closer to 35 Rem.  The ballistic of the 35 Win, at a slightly higher pressure level are only slightly better than the 35 Rem. Before betting any body parts it is good to remember that even the newest Krag action is over 110 years old.

FWIW, you can take any data or advice Para offers to the bank!  And, as Rich stated, regardless of the barrel used, the action is still a Krag action.

Fred, what will most probably happen is that the bolt lug will crack.  That happened (and continues to happen) to many Krags simply by overloading the powder pushing the 220 grain bullet. I did it to my Krag with 41.5 grains of 4350.  Someplace on this site is a link leading to Michael Petrov's experiments in blowing up a Krag.

That said, I have seen Krags converted to both .35 Winchester and .405 Winchester.  The magazine must be modified to take the slightly longer cartridge.

Ken Water's Pet Loads book has chapters on all 3 cartridges; .30/40 Krag, .35 Winchester and .358 Winchester. Walter's work is well worth reading, especially the part about asking too much of old rifles.

FredC wrote on Oct 29^th, 2015 at 12:29pm:

As can be seen in the chart posted below, there is no accurate way to correlate piezo-transducer (psi) and copper-crusher (CUP) pressures. There are just too many variables affecting results in different cartridges.

What I can tell you is that QuickLOAD pressure figures are calculated to correlate with CIP piezo standards, which give these "Pmax" ("MAP" in US terms) figures -

.30-40: 47137 psi
.358W: 58740 psi
.35 W: 44236 psi

If you want to use QuickLOAD to estimate loads for your .35 Krag, measure the difference in weight between an empty case and the same one filled to overflowing with water. That will yield a maximum volume from which QL will calculate usable volume for any given bullet and seating depth. Results will still be guesswork, but a bit more "educated".

Table from a 1987 Hercules data brochure, using SAAMI MAP figures -

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Here's a link to Michael Petrov's experiment:  (You need to Login to view media files and links)

Since it is OK to link to Double Gun this is what got me started down this road a couple of years ago:
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Looking through this thread I saw a reference to 35 krag loads but in recent searches I have not been able to find anything.
Parashooter,
I had already researched the PSI/CUP conversion and found it was not possible to do consistently.
On your chart with the software estimate pressures it has a 250 grain bullet and not the 220 grain I intend to use. Is that correct? If so then 45 grains of varget with a 220 grain bullet should really be in a safe territory. Even if I could get more I like the idea of a moderate charge as there will be less operator punishment and noise.
I have also given some thought to this apparent equality of the 308 30/40 at 40K cup and thought about the difference in the shoulder angle. This will become less of a difference with 35 caliber as there is a lot less shoulder remaining. I have a 308 case somewhere I will try to find it and open it up to verify.
Reincarnated,
You mentioned cracking a bolt with 41.5 grains of 4350? My son's Lyman book shows a load several grains higher at 40K from a pressure barrel. I know different guns will make different pressures with the same load, but I have a theory why some of these old Krags may be developing more bolt thrust with what should be safe loads.
On the barrel that I took off this gun the chamber was .010 oversize for some distance from the rim. Could it be the case can not swell enough at that point to grip the barrel and allows the case to stretch and slide back with too much thrust on the bolt?