Thank you, Bill, for the suggestion of solving triangles. Dale, I'd love to have a look at those spreadsheets, I suppose I know enough Excel. I'd still plan to lift the logic into R, as I find it easier and faster to manage the data+code+plotting (ggplot2) in it for my film tests. I'd be happy to share the resulting code. Having said that, it seems like you're "walking" along the axis, similar what I thought of doing. I wonder if there is some more holistic approach (solving geometric inequalities?) that doesn't involve checking all the points, until a certain precision of the solution has been reached. Many thanks, everyone.