The only way to know is to first use 21 feet. Then measure the cut-off on both sides. So if you get a total of 2 feet, a foot per side, then you can probably get away with 20 feet with a bridge and have about 6 inches to tie off with.
I'm about to string a Pure Storm Tour with VS Touch 16. I'll let you know the results.
Machines: Gamma 6004 2-point w/ Wise 2086 & Babolat Sensor Dual