Quote Originally Posted by thedancefloor View Post

Ath, yes, I'm trying to measure the highlights and blacks at a height that is 2 or maybe 3 stops larger than the small size.
Think of it this way: when you change the magnification you distribute the light over a different area. Double the area = double the time to get the same light per area.
Just work with the longer side of the print as scale - doubling means 4 times the area - open up 2 stops.
Esp. for small magnifications this is an approximation but a good starting point.