I don't know how accurate this is, but it worked for us.
You will see that it will increase in increments of like 1.3 mil, 1.3 mil, 1.5 mil, 1.5 mil, 1.7 mil, 1.7 mil, every level in that order. Just find where the pattern stops at for 40th level, and then go on above it using the same pattern.