How do you get the water out, or if you don't, do you just drain the lower hose
and then add straight coolant to the stuff that's already in there and measure
the specific gravity of it to make sure it is proper?
I drain the now-clean water the usual way, by pulling the lower hose, then add coolant (AFTER reattaching all hoses, etc) such that I would have the desired 50/50 blend based 1/2 the total capacity of the system and then top off with straight water and verify the concentration with a coolant tester after a test-drive for mixing. Also, I usually drain the coolant first, to capture as much as possible to reduce the amount of coolant that runs into the ground when flushing.