What's your kh? If your kh is high enough, it'll help prevent ph swings. My Kh is 3 and seems stable pH wise
Basically, once you know your kh, you adjust the CO2 flow to get the right drop in pH. The drop in pH tells you how much CO2 is being dissolved into the water.
Have a look at this chart.
If your kh was 3.0 and your pH was 7 you'd have CO2 dissolved at 8.7 ppm. For a well lit planted tank you want to aim for 30 ppm, so you'd have to increase the CO2 flow until your pH dropped to 6.4 which would give you 35 ppm
Since you're going DIY, I don't think you can use a needle valve to adjust the CO2 flow, so you'll have to mess about with bottle sizing to get the right CO2 flow.