Reading level of CO2 got nothing to do with DIY, is the CO2 that matters!!
I seldom do CO2 reading when the plants are bubbling (CO2 should be around 10ppm above)
There's a relationship between pH,KH and CO2...
Take your pH and KH reading then refer to the chart at the following website:

http://www.csd.net/~cgadd/aqua/art_plant_co2chart.htm

Alternatively, you can install a CO2 level indicator, a little capsule
with color indication!!
Btw, what type of reactor/diffuser and how low long has your tank been running..??