The manual for the spreader should have that information. You may need a GPS to get your speed, if you don't have a speedometer on the tractor.
Otherwise, you will have to just try different speeds and settings. It's probably cheaper to start off with the flow out of the spreader set low. Then slowly open it more until you get the coverage you want. It's much easier to add more than to pick up seeds from the ground! (Joke).
I like to apply at 1/2 the rate and make 2 passes as 90 degrees angles.
Have fun!
JB