Well, in the "old days", it was 300fps for .68, 330 for .62 and 365 for .50.
The main problem with smaller caliber paint was it had MUCH LESS range, and was far less accurate, so they were allowed higher FPS.
Caliber is not really the issue, but the weight.
Here is a link to find out the safe FPS. (You need to know the weight of the .43, then convert to grains).
Link for energy calculations
First, put in the data for .68, then pop in .43 and walk the FPS up until it matches the energy from .68.
HOWEVER.... 99% of the fields out there do NOT allow higher FPS for smaller calibers. The main reason is they can be dangerous at close range.
Nick