Brake issues, might tie everything together
You are overthinking this to no end. Just fill up the reservoir to the top and then bleed. Once you finish one side go back to the reservoir and fill it up if you are worried.
There is no chance of running dry. I can do all four brakes without inducing air. Actually yesterday I helped a Spyder owner bleed their brake after adding a replacement caliper. No issues and only had to fill it once and it was still to the top.
If you are having trouble with having the reservoir run too low you need to step back and reevaluate how you bleed. This shouldn't be a problem unless you are doing something wrong. In the past I ran into a problem using a power bleeder because it evacuated the fluid too fast and evacuated more fluid then I had to.