basluster alignment and drilling raling stairs.

How can I get my layout precise for a set of stairs that has the top and bottom posts set and the old railing is being reused but is unattached. We are remounting the railing and putting in new balusters that have to be drilled into the top rail and stair treads. I need to know of any tricks to make sure all of my balusters will be lined up when I put the top rail back on. Help!
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Pichers.
Pictures might help, but otherwise I'd say it's a matter of arithmetic.
You lay the rail on the stairs next to the newels. You clamp it in place. You mark out baluster layout on both the rail and the treads at the same time. You mark the length of the rail and measure the angle to cut. Then do it.
Is there some other factor you haven't mentioned which would complicate this?