For a 99H block, standard bore diameter is 2.542". According to haynes, a the dished pistons my car came with would give a CR of 8.3:1
So a 998cc engine is 249.5cc per cylinder.
When compressed 8.3 times, thats 249.5/8.3 = 30.06cc per cylinder
A 20 thou overbore is (0.02/2.542*100%) = 0.78678% larger, so the new volume per cylinder should be:
(1.0078678*249.5) = 251.46cc
Assuming the dish profile is the same as standard, the compressed volume should remain the same, hence the new CR will be:
251.46/30.06 = 8.365 or approx 8.4:1
Now if i were to switch to flat top pistons, minispares says the CR goes up to 10.4:1 meaning the compressed volume on a std piston is 249.5/10.4 = 23.99cc
so if i were to fit a 20 thou overbore flat top piston, the new CR would be 251.46/23.99 = 10.48:1
similarly if i were to go to 40 thou overbore the new volume per cylinder is 253.426cc making the CR 8.43:1 for dished and 10.56:1 for flat top
Of course theres some error here but this should be a pretty close approximation right?