Looking at the dataref as I turn the encoder, the steps for me are not 0.0025, it's 0.0013xx, where xx is variable with each detent (even when using DataRefTool plugin to control the trim input I get the same results). However, this is with the Laminar Cessna 172 REP version. With the standard 172, it's about 0.0022xx, where xx is variable (creating inconsistencies going up and down as it is rounding to 0.0021, 0.0022 or 0.0023). You can never go up one detent from 0, and then down one back to 0. It's always slightly more or slightly less than 0, and the more times you do it the worse it gets.
Is yours actually changing nicely by 0.0025? Or is there an aircraft setting somewhere that can be adjusted to achieve this?
P.S. I tried the Elevator_Trim_Pos (analog input), and it works great as I imagine it working, except just as you said the precision is a bit too large at 0.02. The nice thing is that is always moves by 0.02 up/down, so the movement is very predictable and repeatable. Would be nice to get 0.01 out of it.