When I need to set a precise bit height at my router table, I take two drill bits with diameters that match the router bit height I need. I lay both bits down on the table so they’re parallel with one another and on either side of the router bit. Then, with the router bit raised higher than necessary, I rest a small scrap on the top cutting edges and on one of the two drill bits. I lower the router bit slowly until the other edge of the scrap touches the second drill bit. At this point, I know my router bit height is dialed in correctly.
– Serge Duclos