I have measure other 50' trawler in the yard and they are running from 12" to 24" and some rudder are flush with the transom and some are set back in 12"-16
Be sure and leave enough space between the end of the shaft and the rudder to slip the prop off.
Some years back there was a very angry man
in the yard at Friday Harbor with a bent prop. He had had a new shaft, rudder and strut fitted and the yard that did the work hadn't taken that spacing into consideration.
He wacked a prop and had a diver go down to pull the old prop and put his spare on, no can do!! He ended up hauling out in Friday Harbor to pull the rudder off, just to change out the prop.