I would say 10 degrees should be fine. And that is a pretty short run.

maybe 30 sec to 1 min if that at idle.

The reason I say that is based on what I do.

I have 195 degree thermostate.

So I let the fan come on at 205 to 210. if it takes it down 10 degrees it is right at the thermostate. It actually takes it down to 189ish before it closes I guess and the fan shuts off.

I don't think I would feel comfortable letting the engine get much higher than 210 to 220. Though I am sure I have hit 230ish at some point.

guess it would depend on if you had the turn on point still adjustible.
if you can't make the off adjustible, then I would do 10 degrees and if you want 20 for the switch. but I would be afraid it would never shut off at that point.
guess you could put up a poll.
see if there is a preference to the fan on temp and if they would want it to fight down from a higher temp.