In Hovering The difference between IGE and OGE is nearly 7 % of power / take of mass lose
This rule is totally independant of the altitude...But you need to read the power in IGE first to know the OGE power.
At the same place and weight :
If you have 20 " IGE, you need 20"+ 7% = 21.4" OGE
If you have 75 % torque IGE, you need 75 %+ 7% =80.25 % OGE
If you have 13° pitch IGE, you will need 13°+ 7% = 14° OGE
If you are at 2000 lbs IGE, for the same power, you have to be only at 2000 - 7 % = 2000- 140 = 1860 lbs OGE
anyway what you use : MAP, torque, pitch degrees, weight the rule is "approximative, but "useful"