Exactly, with a combo pitcher/catcher of -4, you can expect roughly 40 stealing attempts, so a T-20 would allow on average 6 bad throws, but with a combo of +4, it's likely 160 attempts or more and 25 bad throws.
Oops, corrections needed...the numbers I gave above apply for -3 and for +3 respectively. I should have added that these numbers apply for 20XX (and even more precisely, for the more recent years 201X), and not for ATG.
To answer bbfan, it's based on my own analysis, based on estimating the impact of catchers' arm and hold ratings.
Here are the details (data for a full season)
Combos averaging -4...22 total attempts...67% success rate
Combos averaging -3...44 total attempts...71% success rate
Combos averaging -2...66 total attempts...72% success rate
Combos averaging -1...88 total attempts...73% success rate
...and so forth, adding 22 attempts at each level, success rate pretty much platoon at 74% for all other levels
I made my system linear (increasing 22 attempts for every "hold" unit) as I didn't see the need to have a steeper increase.
The major limitation is that I based my evaluation on team averages...so these numbers are too gross approximations to hold for individual pitchers. To predict stolen bases attempts for individual pitchers, you would have to consider on-base, ballpark ratings, usage (number of expected innings), etc.