I'm just trying to find an equation that will find out how fast the prop will spin so that I know what prop to buy with the motor

That is the problem, the load of the prop will reduce the RPM from the theoretical (kv * voltage). The motor's torque ability, the prop's drag, pitch, size, etc all effect by how much.

I did some thrust calculations on the Flite Test A pack motor with a 6x3, 6x4.5, and 5x3 prop with 2s and 3s and not one of the combinations created the thrust that it would have given the kv * voltage RPM calculation, IIRC the 5x3 prop's thrust was a higher % of the 'ideal' thrust by the calculator then the 6x3 or 6x4.5 props.

To my knowledge, there isn't a formula that takes into account everything that could impact the RPM for you to just plug in numbers to do this with.

if your trying to figure out what prop to use with a motor, the motor manufacturer should have some recommended prop sizes for the motor, because if you over prop it, you can overheat/burn out the motor as it draws to much current trying to spin to big of a prop. if you undersize it to much, you just end up throwing away thrust (and likely efficency from what I found as, for a given amount of thrust, a larger slower prop tends to be more efficient then the smaller faster prop)