Stamp Measurement Question

As I sorted through some Ireland stamps this afternoon I came upon issues from 1922-23 (Scott 65-76). Then I saw that those same stamps were also issued from 1940-42 (Scott 106-117). The difference is size. The later issues had a size of 18x22mm. Is the measurement taken from perf to perf or is it measured from the outer printed border of the stamp? If it is indeed from printed border then I have the later set.

Any assistance would be gladly appreciated.

Thanks,

Ron