No big issue, I just didn't understand why an excel (or similar) chart was needed because I don't use my phone or computer when in the boat. Too big of a chance of it going swimming.
I see. I wouldn't bring my computer out in the boat either. All I was trying to do was help the OP with his question. Just trying to help him accomplish what he would like to do. Never gave a thought to the unfortunate circumstance of computers going swimming.
Getting a weight between the lines is easy. Simple math.
If only it were. The function underlying the chart is a power function. It is not linear. One will always miscalculate the interpolation if using a linear interpolation ... that is ... except for a single point between the lines where it will work. Not saying you can't do that in a pinch, but it will almost always be a "kind of artsy", incorrect, mathematically wrong way to do it.
