Numerous factors that will vary the pressure drop.
Inlet piping size
Intercooler sizer
Intercooler condition - clogged with oil, bent fins, etc.
Turbo charger condition
Temperature of air going in
In saying that I would go with Alive Tunings 2psi recomendation as it is safer than assuming 3 - 4 psi. So 20psi to 23psi would be the limit to take it to pre intercooler.

