The nail test shows the blade is sharp and the Edge is smooth. Growing up we were taught that test and the shave your forearm hair test.
Back in the 1970s Me and Dad watched the Buck salesman do their classic shave the threads off a bolt then slice up a tomato into nice thin 3/8's thick slices. He got the bolt from the bin in my Cousins Lumber Shed.
I assume you are talking about the drawing across a fingernail test, right?
Also, a 3/8 inch tomato slice isn't very impressive. Thats just under 1/2 inch. 1/2 = 4/8