It's not so much intuitive understanding, but a lot's of years of doing cooler maintenance.
The water at the end of summer in the pan is almost a syrup if it hasn't been changed or diluted.
You should be able to rig up a bleed off pipe from your cooler pan drain, where you can stand on the ground and open a small valve and drain off a few gallons every week, it helps.
A old piece of speedometer cable works well for cleaning your spider.
When you do your cooler maintenance it's best to replace the float every year.
When you fill your cooler with water remove the float and after the pan is full shut off water and install and adjust float, this way you clean out all the loose scale in your water line , and your float won't get plugged up after you are back on the ground and ladder is put away.
Always turn the pump impeller by hand before you flip the switch. Lots of times they seize up over winter and when you flip the switch it lets all the smoke out and we all know that's a bad thing.
Stay cool ---J