Robi, THINK about it!

First trap measurement = hounds to bow tang. Set the trap at that (add some line if you need to)
Second trap measurement = First trap measurement moved aft to where it touches the deck. This is hounds to deck aft measurement and is equal to hounds to bow tang at foredeck measurement.

Where the trap is touching the aft deck will vary according to the amount of rake you have. More rake = touch point further aft.

Measuring this way, it doesn't matter whether one boat has longer wires or not. All you're measuring is relative rake.