Math is your friend

It will be right between 1/3 and 1/2 of the difference in Axle to Crown height. So if the new fork is 30mm taller, your BB will be about 10-15mm higher.
To figure it out exactly:
1. Measure rear axle to BB
2. Measure wheelbase
3. Divide number in line 1 by number in line 2
4. Subtract A-C height of current fork from new fork
5. Subtract sag of current fork from new fork
6. Subtract sag difference in line 5 from A-C difference in line 4
7. Measure headtube angle, take sine of the angle.
8. Multiply number in line 6 by number in line 7(sine of headtube angle)
9. Multiply number in line 8 by number in line 3
Number in line 9 is increase in BB height!
Even that is not perfect, but it is within a couple percent
