Actually, On "some" vehicles (audi for example) When the fluid level drops to the lower fill line, that is a direct indication that it is time to renew the brake pads.
It does not indicate "which ones", but if your pads have been changed all around, when the level indication shows that it's time to "top off", It's really time for a pad slap!
Makes sense to me.
Adiditionally, I like to use a turkey baster to draw out the fluid from the reservoir whenever I do a brake service. No bleeding of air is needed this way , and the fluid is refreshed. I do pump out the caliper fluid contents to get out the gunk first. Water is the enemy of brake systems. Don't forget to lube up the caliper pistons and boots with Sil-Glyde Rust never sleeps!