Hi Rick,
I always used to do it the other way round. Start off going up the three Graig Ddu inclines, then a bit of cross country to above Manod where there is an old footpath (not a public right of way so not marked as such) down to Manod, then head off along the Rhiwbach tramway. It was a full day's walk (allowing time for stopping and admiring the views, artifacts, and taking photos).
The weather can be awful up there though as has already been said.