In this heat wave, a drop of .5 overnight is really nothing at all to worry about. I'm having similar drops in my FC from day to day, sometimes more. With a lot of sun, a lot of heat (my water's usually 90 deg or more), lots of use, I'm not surprised to be adding so much chlorine.

Watermom's advice about getting a higher FC level SHOULD in fact keep your water safe and sanitary even with the drops in FC.

BTW, I don't agree with the advice to only test CYA monthly. I prefer to test weekly. Buying a pint of CYA reagent online is easy and well worth it. During the week, I run an OTO and pH test daily--takes 2 minutes, and I usually use my old HTH 5-way drop kit in the blue box (I replaced all the reagents with the 6-way HTH drop kit--the only difference is they now call it 6-way--and don't give you the blue box!).
But every week I run the K-2006 tests (I use the old PS-233 Ben used to sell--with fresh reagents). I test:

FAS-DPD for FC and CC
pH
Total Alkalinity
CYA
Water Temp.
I log my results all summer.

Since I have a vinyl pool I only test CH once or twice a season.

Carl