Set Effort appears only to allow a number value not a calculated one used in the example.
Which example are you referring to?
What problem are you seeing. I just tried it and it works, other than a speed of 0.25 was too slow for my motors to move the XRP. I needed a speed of 0.4 for it to start moving.
Thanks for your help on this. I was trying to use a Logic block rather than a Math block! Now sorted.