The speed is something you will have to estimate. Figure this way, a trainer flies at about 40 - 50 mph (normal top speed ), a warbird is going to fly at about 70 mph. MANY aerobatic planes similar to the 300 are PROBABLY not going to be flown at much over 70 mph.

To be safe, I would put in 70 mph and see what the calculator suggests, then redo it using 90 mph. The difference in cost between a 75 oz servo and a 100 oz servo is minimal. The big difference is normally the weight difference (unless you are talking digital $$$ ).

Hopefully someone with that plane or a similar one will chime in with a more accurate top speed.