I just pour the 2 TBS of CS in there and fill it up to within 1" from the top.
After I apply the bung I roll it for a few minutes to try to dissolve the suger.
The rolling also builds up pressure on the inside and tests the seal.
As for the remainder in the bottling bucket. That's easy.
(Normal) 4 oz of CS per 5 gals, right? 4 oz / 5 gals = .8 oz CS per gallon.
With approximately 4 gals remaining (I top off my primary to 5.25 gals) X .8 oz that equals about 3.2 oz for 4 gals.
For 3 gals? That'll be 3 X .8 oz = 2.4 oz.