When you are finished the accuracy will only be about +/- .01 inch.
That accuracy is fine, I only need +/- 1mm
Photoshop 7 is pretty old.
CS5’s measurement features may still not be all you would want them to be, though.
In the Scripting Forum stuff like that has come up (mostly regarding scientific use, too) and you might be able to semi-automate the task with a two-point-Path for example (well, provided version 7 already allowed for Scripting, which I’m not sure about).
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In case you need to know how to make the calculation:
1. Use the Ruler Tool to measure one of the rulers in the photo (measured length)
2. Divide the measured length by the actual length of the ruler shown in the photo (eg if you measure a 10cm ruler and the Ruler Tool says its length is 8.5cm then divide 8.5 by 10)
3. Go to to Image > Image Size
4. Uncheck “Resample Image”
5. Multiply the number shown in the Resolution box by the number you arrived at in step 2. Enter that new number in the Resolution box and click ok.
The Ruler Tool should now display the true length.