Correction % = New_ms / Baseline_ms * 100
So, pick a dead time near your average running voltage as the baseline (100%). Let's say that's 14V and 0.8ms dead time. You would enter 100 at 14V in your corrections table.
Now you test at 12V and find 1.1ms. Use the formula above:
1.1 / 0.8 * 100 = 137.5%
Round and enter 138 in your 12V correction cell. Repeat for other lower and higher voltages. I typically enter a range of 8V to 16V values for "12V" vehicles.
Note — you can pick any voltage for your baseline, but I like to see minimum corrections when all is "normal", so those are the baselines I use most often. Just my thing.