Hello Denise
These devices have their uses but you must follow the instructions carefully. They should instruct you on how to compensate for the difference between the down force measured at the jokey wheel and the real force that will be developed at the tow ball.
There have been some reports on this forum that have called into question the accuracy of jockeywheel devices, both in the force indicated and the repeatability of the result.
As it is a legal requirement to keep within the towhitch down force limits for your outfit, you should be aware of the capability and accuracy of the device.