Yes, large/large plus 1" or 2 links is minimum, so if you end up with an odd number you go up to 1-1/2".
You can also work from the other end, threading the RD, looping the small/small and pulling up until the RD begins to take tension. This gives you the maximum length.
Anything that passes both tests is OK. If the chainring and cassette ranges add up to less than the RDs capacity, either measurement alone will work, but if pushing the limits I strongly urge doing both tests, and in no case running a chain too short to safely loop the big/big.
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