How much chlorine should my pool be using if it's running properly?

It's a vinyl liner 18x36', deep end is 8' - I believe it is roughly 25,000 gallons.

Full sun virtually all day, cyanuric acid is 40-50 depending on whose test you believe, not sure of water temperatures but temps outside have been running high 80's to 100's.

A couple weeks ago I shocked and had chlorine levels over 15ppm for a couple days, but since then, both times chlorine has dropped below 5 or so, it seems like it drops like a rock from there - both times, it went from the 1.5-3 range to .5-1 in a day. After the first time, I shocked it back up to 15 for a day or so, but today it once again dropped down to .5-1.

Is this normal?

This evening, I just put in enough chlorine to raise levels up to 8ppm or so. Is that ok, or do I need to shock to 15ppm every time chlorine drops below minimum levels?

Also, it seems like FC and TC are virtually identical when I try to read them on the Taylor K-2005 test kit that came with the house (yes, the reagents are not out of date). Is that typical?

I'm slowly getting the hang of this thing, thanks for all your assistance!