With my friend's clutch, I personally aimed for 87.

I figure my $10 second thermostat with a probe will be 1-2 degree's off, and since 85-89 is safe, I might as well aim for 87 to be directly in the safe zone.

Also, I might as well add that if you incubate at lower temperatures, chances are the eggs will just take longer to develop, although at some point, if you dip too low for too long, the eggs will simply die off.
If you incubate at exactly 89 Degree's, they should be in incubation for 55-60 days, and you should have some nice healthy babies.

If you go the other way and incubate too hot, the eggs will "cook"
Please Note: Do not eat the eggs