[OldCurmudgeon]

AFAIK, The "standard" is 1/32" larger than the actual size. So 1/4" is 7/32", 1/2" is 15/32", 3/4" is 23/32". You can actually buy special "undersized" dado router bits (e.g. https://smile.amazon.com/Whiteside-...619&sprefix=plywood+router+bits,aps,65&sr=8-3) which I found to be close enough for my work.

 

[OldCurmudgeon]

It occured to me that I believe "real" Baltic Birch plywood (sold in 5' x 5' sheets, not the common 4' x 8') is specified with the correct actual thickness. It also has many more layers (I think 13 for a 3/4" vs. 5-7 in regular plywood) and is supposed to have no voids.