I made myself a 60W PSU for my CD32/SX32 Pro using a 60W PSU from an A500 and the lead from an old CD32 PSU, which ended up looking like
this.
I cut the power cable from the A500 PSU (the one with the A500 power connector on the end) inside the PSU itself, and joined the black power cable from the CD32 (the one with the CD32 power connector on the end) inside the PSU using medium-sized terminal blocks. I didn't use a multimeter or anything like that, I just matched the colours of the wires and used a bit of common sense! It worked perfectly, by the way! :-)
Joining the cables inside the A500 PSU gave a nice neat solution, IMHO. 8-)
Steve.