Graham, When I designed my router I incorporated plenty of adjustment leeway into the Z.
I built mine back-wards in the design.
Started out and worked my way in, so I didn't know where exactly my Router was to be positioned.
The yellow and red circles on the pic will give you a ideal of what i did. plenty of mount holes to move up and down. The router clamp slides up and down a dado cut to help keep it squared up.
It's been running good for almost 3 years I think...time to start a new build....this time I will start at the router collet and design out to the X.