Thank you all for your insight.
Does that formula work out pretty well for you? That is what I thought I did once and it just ended up being way overexposed, so I figured it might be a more linear function.
The test strip is probably best, but I can punch in a formula a lot quicker than running more test strips every time I move the head a bit. That is if the formula can be relied upon to give equal resuts. I am certainly not in a hurry to make shoddy prints.