Chuck - Which gauge to use depends on the rifle's condition. If it has a new, unfired barrel, the .067" No-Go is the one to use. If the arm has been fired a bit, the .070" Field gauge is indicated. In addition, it should be OK with good ammo if it passes on an .073" or .074" gauge.
I suspect you don't yet have a solid background in headspace and its effects. You might benefit from the article at
(You need to Login to view media files and links) - written about .303 British but largely applicable to US Krags.
My favorite quote from Gen. Hatcher:
"If there is excess headspace, the case will be stretched; if the headspace is too excessive, the case will be separated . . . where the thick brass in the base of the case begins to thin out as it joins the walls. When the gun is opened, the base of the cartridge will come out with the extractor, but the forward part of the case will remain wedged in the chamber and the gun cannot be used again until this is removed. . . With good strong actions and moderate pressure loads, and especially if shooting glasses are worn, this trouble is not particularly dangerous, but is annoying and inconvenient. Very little, if any, gas escapes to the rear, because the back end of the cartridge, which remains in the chamber, acts as a seal. In fact, it is just such short brass cups that the Germans have for years used as the breech seals in their heavy artillery . . .
"Our riflemen are used to measuring the headspace of their rifles by thousandths, and are prone to become very much alarmed if the headspace gauges two or three thousandths of an inch more than the normal maximum.
This is a good safe attitude to take, but it is something like measuring cordwood with a micrometer. . ."