Here's what I do, pour 1/2 gallon of straight antifreeze
into coolant recovery tank, Label a clean gallon jug
50 / 50 antifreeze mix, pour remaining antifreeze into labeled jug, and top
off with water. Next time system is low, top off with 50 / 50 mix. The "
straight " antifreeze in recovery tank works its way into system over time.
The testers are fine, if coolant is old, and throughly mixed,
test before a top off, not after.