Glad it worked out.
I used MS Visio to sketch lines over the top of a grab from the PDF you linked to.
I could then use the Visio dimension measurement tools for me to read work out scale values. The drawing tools in PowerPoint would help a little, but I don't think they are as comprehensive as those in Visio. Once I had created my Tl, Tr, Bl, Br values for the largest rhomboid shape in the graph, the spreadsheet could extrapolate for any input values. I real challenge would be to bounds check the values against to flag up when the solution point is outside the white area.