Juts thought you all might like to read the "advice" the union person sent me:

The rule of thumd is to ask for up to 1/2 of the fee you received for the original article. However, that is usually tied to a specific time period, to be renegotiated w/in a year or two. To ask for perpetual use should be compensated with that in mind.

Really helpful, hah! I am glad I asked Mudcatters first!!