Is it really 25 x 45? That's a-typically large.

But if it is, you are looking at a pool volume in the 30,000+ gallon range. The most accurate measurement is to read your meter before and after you fill, and deduct normal daily usage.

For now, if you add 3 gallons of plain 6% household bleach to your pool nightly, that should keep you in good shape. Go ahead and buy a cheap local OTO/phenol red drops (yellow/red drops) kit, and use that to monitor your pH and chlorine, that will help.