case volume vs. powder volume

Filling my WWSuper .260 cases to the hilt with BL-C2 powder yields 58.0grains. Filling my Lapua case yields 55.5grains. My pet load was 43.0gr H4350 in the WW cases. Any particular correlation between case capacity and amount of powder as far as how many grains to drop from my Lapua cases to get the same velocity as 43.0gr in the WWcases? I'll try several loads with the crony, but I was just wondering if there was a close calculation for this type of thing.

The Lapua case holds 95% of the Winchester case volume. Use 95% of your pet load.

The relationship between case capacity and relative powder volume isn't going to be quite linear. You need to redevelop the load for the new brass. It may end up close; it may end up nowhere near.