Homebrew PCBs

When I did TT, my process was to print a "calibration board" and go
through all the steps until just before etching, and measure the pcb
itself to find out what the X and Y scale factors are.  Each step along
the way might alter the dimensions somewhat, so it's important to just
check the results you care about.

gEDA/PCB, btw, has separate X and Y calibration values you can fill in
to adjust printouts, just for toner transfer.