What would be just as interesting to look at is on the quantity of fumbles, how many went to the other team? One could argue fumbling 8 times a year but keeping possession is better than fumbling 8 times and losing possession.
I'm pretty sure I made errors below, but I've included both lost fumbles and overall turnovers (just INTs plus Lost Fumbles). It's amazing how Brady turned it around from his early years as far as taking care of the ball is concerned. I generously estimated Montana's ISpTO (they didn't keep track of Lost Fumbles back then), and it's worse than Newton's career mark.
Higher is better for both:
ISpLF: Involved Snaps per Lost Fumble
ISpTO: Involved Snaps per Turnover
Career:
Bledsoe - Lost: 56, ISpLF: 126.8, INT/INT+Lost: 206,262; ISpTO: 27.1
Brady - Lost: 49, ISpLF: 145, INT/INT+Lost: 186,235; ISpTO: 46.8
Newton - Lost: 21, ISpLF: 247.7 115, INT/INT+Lost: 115,136; ISpTO: 38.25
1st 3 years:
Lamar - Lost: 9, ISpLF: 136.8, INT/INT+Lost: 14,23; ISpTO: 53.5
Brady - Lost: 13, ISpLF: 127.8, INT/INT+Lost: 38,51; ISpTO: 32.6
Newton - Lost: 6, ISpLF: 306.5, INT/INT+Lost: 42,48; ISpTO: 38.3
Newton this year: Lost: 1, ISpLF: 288, INT/INT+Lost: 7,8; ISpTO: 36