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If it reads 10.5, DON'T buy it. Sorry, couldn't help myself. I think he wants the range of lbs/sq inch compression/power? stroke to see if engine is rings/valves are sealing well.
I'd do the test on all cyclinders. If the car runs good, and all cylinders are within 10% of each other you should be good t go. Sorry I don't know what PSI it should show.