Using the speed Calc for a twin setup?

[JPriami]

When using the speed Calc what should be done to get an accurate Calc for a twin setup? Maybe increase motor efficiency? Or decrease prop slip? If so by how much? It seems fairly close on all the single drive setups I've done.

[Boaterguy]

you could add 15 to 20% of the single set up, but i'm not sure if that's from using the same motor or a twin vs a more powerful motor.

[JPriami]

In a twin you would use identical motors and setup on each side. Best way I can think is to reduce prop slippage factor. But I wanted to see what others have found that works.