If proportional only, set the proportional band to one to provide a linear output that corresponds to level. The 2500 was available as a transmitter or controller. You should find 2500 series manuals on the web if you want to remove some of the controller elements.
This is a duplicate post that you have on measurement and controls forum.
It is usually frowned upon to multiple post. You already have 7 repsonse to the other post, including response from JL above and me.
I would suggest you red flag this thread, and just go with the other one.
"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?
I think you only need to put the proportional band adjust ad 100% and this give you the transmitter signal you need, you can vary that value it you need to fix another range of level.