A few variables you didn't mention that definitely effect the amount of back twist needed.
1. Loop size.
Are you making the same size loop every time? Be sure you use some type of measurement, whether it be inches of loop, number of twists, etc.
2. Type of loop
Though it is only a small amount, I do put a few more back twists in a longbow loop than a recurve loop(I know that seems backwards, but with smaller loops, you will twist up more of the body of the string than with a curve loop)
3. How far are you twisting down the string?
IF I recall, Chad puts quite a few more back twists in his string than I do, but he also twists up close to 6" more of the string than I do(were talking after you finish marrying the tag ends).
If you're twisting around 8" (from top of loop to the bottom of the twist, i'd try about 15 back twists). IF you're twisting 10 or so inches, I would do around 20 backtwists.
No one will be able to tell you exactly how many back twists to put in the string. Only trial an error can tell you that. But without being consistent on the 3 things I listed above, you'll never be too consistent with the back twists.