Crab, i don't try to convinct anybody. I learned and used this rule since 1977. I just say the difference between IGE and OGE is always : + 7% of power or : - 7% of weight.
This rule is totally independant of the temperatue, the Pressure, the kind of helicopter or so.
BUT only at the same place , At the same time. Please check for you next flight : Power in IGE and then climb OGE and read the power again...I think you read around +7%
Maybe, check your chart and please look the power diffrence at the same Zp, t° Mass in IGE and OGE ....Maybeyou find 7%
Sure, if you read your power IGE today close to sea level and your power tomorrow at 3000ft OGE this rule is totally false.
BUT it's very useful when you work on the same area and maybe limit the number of "SWP"