I use slin pins for the stuff I inject ED mostly. Couldn't say anything about scar tissue, daily pinning is just less of a pain in the ass with smaller needles.
You could do two things: Just order slin pins, only the pin, with a Luer lock (or whatever you use) and put them on your barrels as normal. Considering you want to know how to measure properly, I'd assume you want to use the one piece slin pins. First off, you'd have to backload them. It's not an issue. The measuring process is pretty simple. Most of them only hold 1cc anyway, that's 100iu of slin. So .25ml would be 25iu, .33ml 33iu etc.