260 rem brass to Lapua

You can weigh the cases (preferably both new or clean) use the difference as a percentage and subtract it from you current charge plus 1 grain for safety margin and work up.

Example, Case 1=175gr and case 2=168gr, 168/175= 96%. Say your current charge is 43gr, multiply that by .96 and you get about 41gr. Subtract 1gr and work up.