Using Ph Drop Method To Ballpark Co2 Concentration Problem

I have ph=8 - 8.2 degassed tap water

My kh tested is ~5(API), although a municipal water report says my carbonate hardness should be between 6 - 7.2

https://barrreport.com/threads/co2-ph-kh-table.10717/

I thought this chart would help me and could only overestimate the CO2 in my water but now I am not so sure.

I have injected CO2 into a 17g tank and gotten to around ph=6.6. Given the inaccuracy in my API pH test kit I'd put my ph drop to be max 1.7 and min of 1.3.

1) What would be a realistic range of CO2 values that I could expect?

2) Does the additional basicity of my water (I don't know its source) mean I will underestimate(not overestimate) the CO2 dissolved in the water?

@Tom Barr I would be greatly appreciatve if you would be willing to give me some insight on my situation.