I think the best way to predict sinker burn times is to check the jobs you've done so far and how long it took and then try to categorize them as good as possible; so you, or your boss, can use this information when he's estimating prices:
(A) Material of the part:
(B) Width and height of the electrode, Or; Area of/in effect simultaneously:
(C) Material of the electrode:
(D) Electrode purpose: Rough/Finish
(E) Depth of the sinking area.
And when you've got all this information you can start figuring out some kind of formula for the feed per minute relative to (A, B, C, D and E).
Theoretically, It's the total area of the electrode that is in effect (simultaneously) that determines the EDM sinking burn time, This of course is relative to the information of (A, B, C, D and E).
Unfortunantly, Theoretically and EDM-machining doesn't seem to like each other that much so it's hard to actually predict anything, Especially since no machine seems to EDM a part as fast as another.
Maybe you should check with the manufacturer of your sinker, They should know how to calculate this stuff, Especially since they built the machine.
However, I know there's some researching going on about EDMin, Concerning feed per minute and why, And also how to determine it. I don't know how far they've come, though. :-P
Ah well, I hope i gave some useful information.
Good luck and take care! :-)