We usually wait until they drop below 5o on a human meter (68 on a pet meter) and give a couple of drops at a time with a little food and then test again in 20 minutes to make sure the numbers are coming up.
Once up, you continue testing for about the next 2 hours because they can drop back down again....you want to make sure they stay in safe numbers without having to feed them anymore
Just an FYI...the prefix "GA" stands for "Gone Ahead" or "Guardian Angel"....we use it for kitties who have passed.