For a 75 gallon, I highly recommend a Cerges reactor. You will find that it's a bit more challenging the other ways. As @Sumson has mentioned, those methods are okay, but measuring pH drop seems to be the most accurate way to measure CO2. You want a 1 to 1.5pH drop from degassed to gassed. So you will take a sample of water from your tank, let it sit for 48 hours then pH it. That is your degassed pH. Next, take a water sample after CO2 has been on for about 2 hours, pH it. That is your gassed pH. You will want to measure a difference of about 1 to 1.5pH.
I would start slow, around 3 BPS, after 30 minutes, take a pH. Then double the BPS and check again in 30 minutes. Repeat as necessary. When you get close to a 1pH drop, start watching the fish. Increase CO2 a bit slower and watch the fish. If they start sipping the surface, you've gone too far. Just back it down a bit and you should be on target.
Here's a great resource for CO2 101.
https://www.2hraquarist.com/blogs/choosing-co2-why
The biggest mistake people make is they don't push enough gas out of fear. Just take your time and get it right, it makes a big difference.