My feeling is that every time you read a ruler you have the chance to make a mistake. That’s why I love using brass set-up bars whenever I can. By feeling from the top of the cutter to the top of the bar, I know the bit height is correct. This set includes five sizes; 1/8″, 3/16″, 1/4″, 3/8″, and 1/2″. I use them all the time to set router bits and dado heads to the right depth, and to set the distance between the fence and cutter. You’re not limited, dimensionally, to the sizes of the bar stock. You can stack bars to get “in between” sizes. For instance 1/8” + 3/16″ = 5/16″.
Once you start feeling your way to set up, instead of measuring your way there, you’ll never go back, and you’ll wonder how you ever worked without set up bars.
Whiteside 5 Piece Brass Set-Up Gauges