IMHO, laser particle sizing is still the gold standard, despite issues with grind particle shape.
1. Abandon your spreadsheet.
It is unlikely you will find a linear relationship between grind setting, particle size distribution, dose, and flow rate. A certain amount of trial and error is part of the espresso process. Get used to it.
2. Pick a reasonable dose for your basket (e.g., 18g for an E61 double basket).
3. Adjust grind to achieve the desired desired brew ratio in 25-30s.
4. Vary other parameters (coffee dose, brew temp, extraction time, etc.) until you find a sweet spot for taste. This may involve further grind adjustments.
5. Repeat for each new bag of espresso beans.
For example, if you want a brew ratio of 50%, adjust the grind (by observation/trial and error) until you extract 36g in 25-30s. Ideally, blonding should occur at roughly the same time as your shot terminates.