Your method will include the distance from the center of the nail to the center of the pin hole. Say a roller pin is 1/8" diameter. You use a 1/16" nail. If the chain is hanging from the nail , the nail center is 1/32" higher than the pin center so your whole chain now shows 1/32" stretch before you even put it on the bike.

Now your appproach will work well if you ust a nail that just fits inside the chain and you nail it exactly square with the wall. (You will have to cut off the nail head to get the chan on.) You are giving me ideas. I may well install such a checker on my shop wall. But if I can find the place, I will go twice as long, 24". Maybe even longer. 48" would work well for derailleur chains (but not short chainsay fix gears; rats!) If I went 36" to have one system, I could use 1/16" - just fine, 1/8" - take off bikes with newish FW/cassettes/cogs, 3/16" - take off but save as well worn. 1/4" - toast/art projects.

Ben