My 1898 Krag is equipped with the 1898 sight, with both windage and elevation adjustments.  In addition, my sight has the flip up .04" diameter peep sight. Was this peep sight, because of it's very small aperture, good only at ranges further than 100 yards (200+) ? Is it practical at 100 yards? Thanks.

It's not the target distance but the ambient light and the shooter's eyesight that make a small aperture difficult in some conditions. Of course if the bullseye is so large that it fills the entire aperture, aiming is hard - but we normally use small bulls at short range and larger ones farther out, so perspective makes the apparent size similar.