You will most definitely need an unspooling adjustment.
The calculus is muddied because the spool does not single stack. Fortunately, there's an easier way.
- Unspool 1 rotation and measure how much was unspooled and record
- Continue unspooling 1 rotation at a time, measuring the amount unspooled, and recording, until you have it all unspooled
- Store these values in order in an array
- Take your number of tick from the encoder, divide by 175, plug that value into the array as an index and you have a very accurate measurement, likely to be accurate to within 1 rotation's length.
To increase accuracy you can
- Modulate your reading by 175 to get a remainder from the division
- Take the value from the index above
- The value from the same index - 1
- Subtract the value from step 7 from the value from step 6, to get the value of the current rotation
- Multiply your remainder from step 5 by the value from step 8
- Divide that by 175 to get the measurement between rotations
- Add that to the value from step 4 and you will have a very accurate measurement
I know it reads like a set of stereo instructions. I will better illustrate later when I have a moment.