The theoretical necessary contact force is based on multiplying the cross sectional area of the sprue bushing or nozzle opening (whichever is largest) by the injection pressure (in plastic pressure).

In most cases, the only critical force acting upon the nozzle is the pressurized plastic. Mold deflection will push on the nozzle, but it actually helps to create the seal between the nozzle and the sprue bushing while the actual plastic pressure pushes the nozzle away from the sprue bushing.

-Andy