Dunder giving you some good advice. But, reading your initial post....seems to me somethin'' is off. When I strip UJSM (5 Gal of 10% wash) I get back 1 Gal of 30%. And that's runnin it down to a nub. You say you got 2 gallons stopping at 50% off the same thing? That would be more alc in total than you started with in a typical UJSM wash.
Like dunder says, you run it till it ain't worth runnin it no more — and for most folks that's somewhere down to 10% or less on a strip run. It's mainly a "volume" reducer to concentrate the alc (and flavors) to prepare it for a 2nd, higher, distillation. If you just keep going, the vapor temp will eventually reach the boiling point of water (212F) and it will start rendering H20 back from the pot...dilluting your alc until you'd eventually end up with the abv you started with if you just keep going. So, you have to stop somewhere. And if you notice..once it approaches water boiling point (or zero percent alcohol) things really slow down a lot. Rather than waste the resources (heat/water, etc) to get that last few percent of alc...most people quit. On a standard 5 Gal UJSM wash (around 10%) you should end up with about a gallon of 30-35%.
All-in-all, it sounds like you are figuring it out (ie., you were stopping too early). The other problem with backmashing into your fermenter using pot leftovers that still contain higher concentrations of alc is...that it will eventually stress your yeast and/or kill them. Given the proof of your "strip run" and the amounts you collected, I'm wondering if you aren't somehow combining your low-wines with wash and running that again??