4 pucks per week equivalent to 1.5 lbs Cl2 gas
PF=6.7 for 18k gal pool

So 1.5 x 6.7 = 10 ppm chlorine per week.

Typical daily use, on a lightly used pool in a sunny area, when everything is running well is 1.5 - 2 ppm per day. 10 ppm is at the bottom end of that.

IF there is ANY algae, or ANY problems, chlorine use will be more on the order of 4 - 5 ppm per day OR HIGHER. Cleaning up algae can consume 100 ppm of chlorine in just a few days.

You need to MAINTAIN a chlorine level of 10 ppm (based on your CYA), doing what ever is required to get there. Once the pool is spic and span, you may find that 4 tabs per week (or 2 gallons of bleach) may be adequate to keep it there. But that's not the case, yet.