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.
I don't think that's quite it, but it's close. I don't think this is a case where we can use 30 as the ratio between each chain length; empirically it seems to be off, at the very least--I've estimated it as closer to 12 by looking at the gap between 1- and 5-chains.
Also, that would not be the blocks per hour; it would be the number of 8-chains per hour. Primecoin has a fractional difficulty that is roughly linear, so if the difficulty is 8.5 then about half of all 8-chains are not valid work shares. If the difficulty is 8.99 then only 1 in 100 8-chains are valid. You need to account for that before you arrive at blocks per hour. Generally it is blocks per hour = 8-chains/h * (1+difficulty-floor(difficulty)), but when you get close to the next integer difficulty things get weird since 9-chains work for difficulty 8.XX and 9-chains are more common than 8.995 chains.