Which 7mm-08 Ackley? That can mean various things. The real question is, how much greater case capacity will it have over the 7mm-08 using the same brass as its basis?
I suspect that the increase will be no more than 4.0 grs of H2O (or now equal to the 7X57). For published 7mm-08 loads that are compressed, you can remain with enough powder for the case to be from full to just compressed. For faster powders (where it's not full to compressed), add up to 3.0 grs. Without pressure testing equipment, you can't be more scientific than that.