I believe the mag check at 75% is for the same reason as the R44. When selecting left/right mag you can accidently switch the engine off then quickly turn the key back to both which damages the transmission due to the excessive snatching (torque) of the engine on restart. 75% reduces the torque hence less chance of damage to the transmission should you accidently switch off then back to both. obviously if the key switch is turned to off the engine should be restarted in the correct sequence
Stuart