If you're overheating, see whether or not it'll take more coolant by repeatedly filling to that return hose. It takes just one or a few hot/cold cycles for the level to lower. Any excess will simply be pushed out via spillway underneath the lid. (you'll smell it while driving)
I tried to perform a coolant flush this past Winter. I used the word, "tried", because following the manual, didn't get much out due to the design of the cooling system. But what I noticed on "refill"... which was actualy more like a top-off is, I could keep adding for quite a long time *if* I kept filling it up to the top-most hose... over and over and over and over and... If I just below that tube, wouldn't move; or at least would take wwaaayyy too many engine cycles. The system is going to cool much less efficiently if not truly full.
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