Your chlorine additions can give you an idea but liquid Chlorine degrades in potency so using acid/ph to gauge results generally has less variables. Also, the tub consumes fc around the clock so if you want to verify your dose do it 10 -15 minutes or so afterwards - not tomorrow.
Is much FC used when the cover is on? I know UV consumes FC and I was under impression of a cover on hot tub is used, the dissipation rate is very slow.