Compute horsepower from torque and engine RPM (HP = Torque × RPM ÷ 5252).
5252 falls out of the unit conversion between torque in pound-feet, rotational speed in RPM, and power in horsepower (it comes from 33,000 ft·lb/min per HP divided by 2π). It's a fixed constant, not something you adjust.
Because of this same formula: when RPM equals 5252, HP and torque (in lb-ft) are numerically equal. Below 5252 RPM torque exceeds HP numerically, and above it HP exceeds torque — a well-known quirk of the identity, not a coincidence of any particular engine.
No — torque must be entered in pound-feet (lb-ft) for the HP formula to be correct. If you only have torque in Newton-meters, the result card's "Torque" conversion shows the relationship, but you should convert N·m to lb-ft (divide by 1.35582) before entering it as input.