Simple formula
Try this
n = 0.2 ρ b ℓ R UČ / Mg
where
· ρ is the air density in kg/m3 (1.225 in ISA SL conditions)
· b is the number of blades of the main rotor
· ℓ is the average chord in meters
· R is the main rotor radius in meters
· U is the main rotor tip speed (Ω.R, where Ω is the rotor RPM) in m/s
· Mg is the helicopter weight in Newtons
This should be valid in the 80-100kt range where you can reach the maximum load factor. It decreases at highre speeds.