You're a glutton for punishment,Dibs.
The way to centralise the router on the plate and finds its mounting holes is as follows....
Place the router plate on table or a piece of stiff card? draw round the plate, as the plate is rounded off on its corners make these corner square on the table or card template.
Draw straight lines from corner to corner ie X marks the spot,in this case the center position fo the router,stick a V bit in router and position this bit right smack bang in the center,align router base flat wall if any? with edge of table /card so that its equal.
Draw round router base and remove router,take off routers plastic scratch plate(Some are glued,some are screwed down)place this plate ove drawn router base template and this will then give the position of where to drill the mounting holes in the plate.