One thing to note is that your pool looks to be less than 30K gallons and more like 25K or so. Not a big difference but enough that if you are experiencing any difference between what you expect to happen with chemical additions and what is really happening, that could be part of the reason. You would be seeing more of a change than you expected if you base additions on 30K gallons.
I calculated it as 18'*36'*5.5' (average depth)*7.5 (gallons per sq. ft.) which gives 26,730. Then adjusting for angled sides and the fact that most liner pools are proportionately not the average of the shallow and deep-ends because the hopper is very angled and the shallow end tends to be flatter than gunite pools with deep-ends, it has to be less than the calculated number by some amount. If you are adding bleach by the gallon and doing the FAS-DPD test before and pretty quickly (within reason) thereafter, you can use the Pool Calculator in reverse to help you zero in on what size your pool is too. So long as your chlorine is not being used up extremely fast. You can then see if that jives with what # of gallons you think the pool is by a stated amount or calculated amount.
Our BJ's (warehouse club) carries liquid shock (bleach) in the 12.5% strength for $13.99/case (4 gallons). You may want to check alternate sources for your bleach at cheaper prices. There is also a thread somewhere discussing only bleach prices.
I enjoyed (maybe not the right word) following your balancing act and hopefully you will be there soon.