Thanks gentlemen. Finding a the forward limit on the sloping forward face for a given weight is exactly what I'm trying to do. It's for a basic W&B spreadsheet my new employer has done. At the moment he has an image of the graph inserted into the spreadsheet. I'd like to replace that with plotted & computed points. By testing for the forward limit at the computed weight I can use conditional formatting in the data entry cells to warn of an exceedance.
The forward limit is a straight line because it's an arm:weight graph so equations for straight lines are appropriate here. Mathematical rebalancing used to be part of the CPL syllabus & I used to teach it so it's a annoying not being able to remember how!
My notes are half a world away so I can't look it up.
I'm having trouble working out what the '+ b' part should be in the formula X=Y m + b, where X is the arm to be found, Y the calculated weight, m the slope & b a constant: Is it the distance up the Y axis where the line would intercept at x=0?