but can't remember the "rule" or where I read it.
As Susan covered, the convention is the neg has at least half the ISO rating of the pos. Shooting to expose the positive correctly, makes a negative too dark to use. Shooting to make a negative usuable gives a positive so overexposed it is a throwaway...
It depends too of course on the results you are looking for, so experiment, experiment, experiment - just the thing you want to hear when its USD2+ a shot eh?
Regardless, when I shoot Type-665 (rated at ISO80), I shoot for the negative at ISO50. Pity Type-665 is on the way out, at least we'll have Type-55 (which I've not used, but the rule for ISO50 would be to shoot at ISO25-40 for the neg as CP suggested).
Other info: the fixing stick should be used if you want to keep the pos from yellowing, but with low contrast results on the pos side, and generally overexposed, I hardly use the fixing sticks at all. They do however get used for fixing expired film color prints (which never seem to dry) as they leave a nice streaky finish, and pop the colors a little.
Also, the negs solarize rather well. So if you're shooting outside, you can wait for half development time (or similar depending on the effect wanted), and peel-apart letting the sun burn the negative. Different washing options for the neg too add to the fun.
(These are all Type-665 shot at ISO50:
http://www.filmwasters.com/grabs/v/15_Feb/Skorj/ )