Is it a bad plan to recon my Reta with just 0.5 mL of BAC?

Nothing wrong with that approach. I also keep my injections at 20 IU or below by adjusting the reconstitution volume. For my R50 I use 1 mL BAC.

No. There is a theoretical concentration limit for solubility, but 0.5 ml for 10 mg stays well below that point.

It's mainly a mental hurdle that can be overcome. I've done 80-unit injections several times and they don't feel like anything special. If you picture a teaspoon from cooking, 80 units is only about 16 percent of that.

Staying within about 10 percent of 0.5 ml makes only a tiny difference with Reta. For some other peptides where precision matters more, tighter accuracy would be needed. At 20 mg/mL a 10-unit draw is 0.2 mg; a 10 percent error in either the water or the syringe measurement gives roughly 0.02 mg per unit, or 0.2 mg total at 10 units. That level of variation seems acceptable for Reta.

I use 0.6 ml for my t30 and draw 30 units each time.

Has anyone figured out a practical lower limit? I have some T10s that may sit until they become single doses. In that case I would probably try 0.25 mL, but any guidance would help.

I don't see the issue. I take 6 mg twice, one for myself and one for my wife. It only takes a few extra seconds to push the plunger. After one more 6 mg dose I might switch to a larger vial that would require 36-unit pins.

Draw 1 ml into a syringe and empty it into a spoon to see how little it actually is. I use an injectable medication at 100 units every two weeks and the volume difference is barely noticeable.