Thanks,
@Morgan. I will no doubt investigate these. The app for my iPad is appealing (see what I did there
) but I never have enough storage space, so every time Apple releases an update, I have to offload what I can. Schematics appear to take into account vertical and horizontal measurements. With a very rounded back, I do not want to lengthen the whole of the back, just the centre, tapering out to the sides as one would for a round back adjustment in a sewing pattern.