The filler will depend on the size of the cap.
For example if you have a 00 Cap-M-Quick you need to figure out how much filler can fit in the cap. I usually weigh out one cap (which is usually .13g - I think, the batteries in my scale just died) then you pack it with filler. Weigh the entire thing and then subtract the weight of the cap.
Now, lets say it will hold 750mg of filler for each cap. Subtract 20mg because 20mg of the AAS will be in there. Then take 720mg times 50 caps which gives you a total of 36,000mg or 36 grams of filler.
In the end you take 36 grams of filler, throw in the var and mix it up real good (real good otherwise you will have uneven dosages - it would suck to take a cap with 200mg of var in it). Then simply pack the caps and you have 20mg per 750mg cap.
I hate capping powders because it is a pain and the issue with having dosages that can be way off.
I have a scale and I measure out each dose. It is easy because I bought these spoons. The smidgen size can me used to eyeball a near accurate dose which I throw on the scale just to be sure. They cost $3 at Bed, Bath and Beyond.
http://www.spoonsisters.com/Merchant2/merchant.mvc?Screen=PROD&Store_Code=100&Product_Code=15916
Oral solutions are also an option - see Kian's post in the chemistry forum.