Forgive me if this has been asked before... but what is the absolute minimum for fall (not worried about natural vac, but would be nice)?

Residential setting with total mainline (1/2") distance of under 100' (more like 88-90') with laterals branching off another 30-45'.

Will be tapping a little higher on the trees with the laterals (maybe just under BH). I know I will need the 55 gal barrel below grade (some), and will be pumping into above grade IBC for larger capacity (330gal)) but would like to minimize my digging based on your suggestions.

1/4" per 10'? 1" per 10'?

Also, I have read something about this on another post, but cannot find it. What is the easiest way to tell fall (over 90') without a transit? Was thinking about getting my wire strung and in place then take a simple inclinometer and put a few º on it.