I don't think it's a good idea to apply a blanket factor across all props like the spreadsheet programs tend to.

I have noticed that loads can vary greatly between brands in some size ranges and the gap can close or the situation can reverse when the same brands are compared but in a different size range.

The Master Airscrew black props and APC props are a perfect case in point. Take an MA black 11 x 6 and an APC 11 x 6 and try them out. There's an enormous difference in load between these two - upwards of 2,000 rpm (the APC is a heavier load). Now, get 12 x 6 props in each brand and compare them. The MA black 12 x 6 and the APC 12 x 6 are to all intents and purposes the same static load as each other. Get a 12 x 8 in each style and the APC imposes a much greater load again. There's no consistency at all, it seems, when comparing MA blacks against APCs.

A blanket brand-specific coefficient does not apply to these prop load examples, hence my general cynicism relating to the use of them.