Hi Alasdair,
Sorry- I was thinking you meant something different. In Acrobat X, activate the Measuring Tool and with the cursor on a blank spot on the page Right Click (not on a line you've drawn but in a blank spot on the page with the X cursor for the tool active). Then you will see the Right Click Content menu with Change Scaling Ratio as the top item.
Hope this helps,
Dimitri
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