This shouldn't be taken as 100% correct, but I seem to remember it being mentioned that each step up in chain length is a 30 fold increase. so an 8 length chain would be 30x more difficult than a 7 length chain, and a 9 length chain would then be 900x more difficult than a 7 length chain.
So in other words, your average rate of blocks per hour would be the 5-chain rate / 30^(diff - 5). Time to write a script...
Edit: updated
the script.
Why did you write this script? what did you change, and what problem did you solve?