ff_speed = ((log(whorld_speed) / log(20)) + 1) / 2

where the nominal pitch is 1.0 (100%), half-speed is 0.5 (50%), double-speed is 2.0 (200%), etc.

Some examples:

whorld FF

1.0 .5

0.5 .384311

2.0 .615689

.6666 .432326211

.27 .281466900

Time Blur must also be compensated. The solution turns out to be trivial: just divide the Time Blur by the pitch, i.e. if Time Blur is .27, and pitch is .5, Time Blur should be .54. The idea is that if you're creating half-speed data, you want to blur twice as many frames to achieve the same effect.

Other Whorld parameters:

ff_zoom = (log(whorld_zoom) + 1) / 2

ff_hue = (whorld_hue / 360)