Well, according to the calculator:

- 14mm root diameter (subtract 2mm due to thread size)
- 1800mm length between bearings
- B-style end fixing (one fixed, one floating)

Maximum speed is 522RPM.

Even with a 16mm root diameter, it's 597RPM.

(But I know engineers like to put some extra for safety, so it probably would work up to 700RPM).

Or do you have another way of calculating it?