Pro-rated percentage increase formula - help.

I need some help creating a formula to calculate a percentage increase for an incomplete pro-rated agreement.... Ok, that probably doesn't explain much! Let's try again:

I recently agreed to do a pre-determined volume of bulk work over the next year at a discount. To keep things simple, I offered a 40% discount from my regular rates on the assumption that I would be paid for all the jobs on a monthly basis. However, there is the chance that my contract could be terminated early. In such a case, I want to be paid a pro-rated percentage on the volume of work completed so far. For example, if I had completed 50% of the work and was fired, then the client would only get a 20% discount for all the work done up to that point.

The formula that I came up with goes like this (let's assume there are 300 jobs in total and I completed 150, or half, before the client cancelled the contract): 150/300*40%=20%. So in this case, because the client had essentially only paid me for half of the original full bulk contract, they would only be entitled to a 20% discount from the normal rates (rather than the 40% discount which applied only to a full bulk contract).

NOW, this is all nice and we can do the reverse math by multiplying the original price by 20%, BUT I've already offered a pricing list which has all the discounted (40%) prices. Essentially, I want to offer the client a formula that they can apply to the discounted rates that they would have paid up to whatever point I would theoretically get terminated (that is, the full amount of money I had collected upto that point) so they can easily determine how much they owe me for the early termination. I can't simply multiply the already discounted rates by 20%, for instance... I hope this makes sense. Ideas anyone?

By the way: if this post belongs in another thread/forum topic, please let me know which and I'll move it.

Thank you!