I think the original problem I had was that the reproduction on the photocopied load sheet that I was using (not the one linked) was so small and fuzzy that it was difficult to establish the horizontal spacing of the 'TRIM' units with sufficient accuracy to enable reliable results, giving me somewhat inaccurate results. I guess I should have scanned it and enlarged it. As you correctly point out the spacing is in fact as near linear as you can tell so an equation
should have worked, given the correct figures. By accepting that the result from the original paper sheet I used to use was probably in itself a little 'fuzzy' I am no longer striving for the decimal point accuracy and - job done. Thanks for the nudge.
