I cannot access the tire gear & speed calculator

MPH= (rpm x Tc)/( 1056 x R1 x R2)
Where Tc is tire circumference; ~diameter x 3.1416
R1 and R2 are your gears; trans and diff.
Results are for zero-slip, like a manual trans or loc-up.
With an automatic, you have to add TC slip which varies with roadspeed and throttle, and can be different in every gear.

If you need rpm at a certain speed; rearrange the elements like so;
RPM= (mph x 1056 x R1 x R2)/Tc
Again, for an automatic, you will need to add TC slip

As to slippage;
If you think about it, idling in gear with an automatic, and stopped, no matter how you slice it, if the car ain't moving, that is 100% slippage.
Whereas at the top of the track almost on the rev-limiter, the slippage might be less than 10%; so the tach will be reading, some slippage amount, higher than it would if the car had a clutch with zero-slippage. But I bet you knew that.
At 65 mph, you cannot add an arbitrary fixed rpm, for all TCs. Cuz the slippage will vary due to the amount of torque input versus the vehicles resistance to forward motion, and how close you already are to locked up. But you can use a reasonable slip factor like 3 to 5 percent, perhaps as high as 8% with an old loose convertor.