Best time is probably late on in the day as there should be a good sunset tomorrow, as long as the har doesn't appear.
At that time, around 9.30 to 10.00, South Queensferry is your best bet.
During the day, a trip to Port Edgar gives good views. You can park ( for free) almost under the Forth Road Bridge if you turn right at the entrance to the harbour/marina.
The approach road to the harbour goes directly under the road bridge and there is a stopping point there; interesting view of the underside.
You can also see the work on the new Forth crossing further up the Forth.