Hi,
BEFORE YOU DO ANYTHING ELSE-- you need a proper test kit, a Taylor K-2006 or K-2006C or something similar with an FAS/DPD chlorine test (not simply DPD). Without it, everything else is a waste of time and money.

Here's MY rule of thumb for bleach:
1st, you have decide what level of FC (free chlorine) you want, in ppm (parts per million--the standard measure)
2nd, you need to know the concentration of the bleach you are using.
3rd, you can use the formula or the bleach calculator to determine how much you need --(1,000,000/pool gallons) * concentration of bleach * gallons of bleach = FC in ppm
4th, or you can use my rule of thumb: 1 gallon of 6% bleach adds 6ppm to 10,000 gallons, or 3ppm to 20,000 gallons or 12ppm to 5,000 gallons. 8.3% adds 8.3ppm to 10,000 gallons, 4.15ppm to 20,000 gallons, or 16.6 ppm to 5,000 gallons. Adjust similarly for different concentrations and pool volumes.
Other chems are sold in fixed units. But they are easy to get.