As far as substrate I use 6 cups in a 7 quarts tubs

As far as your temperatures there are several things to take in consideration when setting up your incubator

1# Having a RELIABLE t-stat
2# Probe placement (I place mine in a dummy tub set like any other tubs)
3# Preventing hot spot by having fan(s) moving the air.

You really want stable temps yes you can have a variant depending on the tub location (top to bottom) however temps should be stable ideally between 88 to 90....you can incubate as low as 86 or has high as 92 however I do not recommend such high temps.

I incubate at 88.5