I've loaded 125's the same way I load everything else....put FFF Goex to within 1/8" of the top of the case and seat the bullet. Now, I shoot CAS (which I'm thinking you do as well), so I'm loading these for my rifle....which requires an OAL of 1.50" to feed well. The only reason I'm putting powder to within 1/8" of the top is to (1) try to keep from spilling powder all over the place and (2) insure there's no air space after seating the bullet.
Hope this helps.
T6 a/k/a Chickahominy Charlie