3 5/8 drams = 99 grains. 3 3/4 drams = 102.5 grains. Did he state what brand of powder he was using. Data in a Lyman Black Powder handbook did not meet these figures, but were slower.

A modern day 3 3/4 DE-1 oz load is supposed to give about 1330 fps, so this is indeed right in the same range. Some would depend of course on the exact placement of the screens which would vary the velocity a bit if at a different distance from the muzzle.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra