Peter,
It is simply the magnification equation...
Enlarger Negative Stage, Lens & Baseboard Distances => u=v*F/(v-F)=F*(M+1)/M or v=u*F/(u+F)
where,
u = lens to negative distance (inches)
v = lens to baseboard distance (inches)
M = Magnification
F = lens focal length (mm)
u+v = negative to baseboard distance (inches)
I have a spreadsheet where I put that into a single sheet, and ran some numbers for different focal length lenses to determine magnification, given that my enlarger head was hitting the bottom of a floor joist and constraining the maximum u+v value even with the drop table.
Good luck with it,
Len
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