Estimating fracture pressure (looking for literature to support)
fracture formula What is the method of injection and fluid you are using? Apply a leak off test to the formation after drilling out casing shoe and your 5-10m into open hole. I’ve attached two links that may be helpful.                           formation fracture pressure
                         a list of methods
There are also plenty of apps for you’re phone that you may find useful. Search “drilling formulas”

Basic frac graduent is 18kpa per meter there fore at 1000m it would take 18000kpa pressure  more or less to fracture a zone.
In psi it is .8 psi per ft of vertical depth.
Usually it takes a higher pressure to frac a formation but feed rates are much lower.

Kind regards
Jeff B.