Mjr
Is that the track which goes past the Old School house?
At Ullapool, it's last on the right before crossing the river heading north, signed as "Breedon Morefield Quarry" on streetview.
It goes past the quarry, Loch Achall, East Rhidorroch Lodge and Loch an Daimh and The Schoolhouse at Duag Bridge. From there you can either head for Oykel Bridge (~ 4 miles) or head down Strath Cuileannach to Croick (tarmac starts) and Ardgay.
There's a fork in the track above the W end of Loch an Daimh - the better looking track drops down to the Loch and doesn't go anywhere; you want to keep the height.