is asking about a "bit", so I assume he/she wants to use a router.
I don't have experience using a router for dados. Based on a comments above, I assume that you must have a router bit whose width is an exact match for the plywood, and plywood width is not always predictable.
I use a table saw and a Freud SD208 dado set. I would measure the plywood with calipers, configure the dado set, run a trial cut on scrap. and finally adjust the dado set width if necessary.
If you have a table saw and decide you want to buy a dado set, be sure to read the table saw manual first to make sure it is compatible with a dado set that is wide enough for your plywood. You may need a zero clearance insert and maybe a special washer or other fastener. (My table saw uses a special bolt for wide dados, for example.)
Regardless of whether you use a router or a table saw, first run a test cut on scrap to see how it fits the actual plywood. After that, you can adjust accordingly. If you are using a router, hopefully you have found the correct matching router bit. Good luck!