to the original question of calculation of 10-85 slopes, I would use linear referencing in the following general geoprocessing:
1. calculate a field in the drainage line/flowpath that is from, and set it equal to zero
2. calculate a field in drainage line that is equal to the the shape length of the stream
3. calculate a field in the drainage line that is the shape length *0.1
4. calculate a field in the drainage line that is the shape length *0.85
5. make sure you have a unique identifier for the drainage lines
create routes from stream lines (uniquely named) using the digitized direction (correctly drawn) using either the projected from (0) and to (shape length) calculated into fields;
export the table/make table view from the drainage lines and make route event layers as points of the 10% and 85% coordinates along the stream route
calculate elevations at those event layer point locations, one way is using "Extract Values to Points"
The event layers should cleanly join back to the drainage line unique identifier/route identifier, so then
calculate slope =delta elevation/delta 10-85 coordinates along the route
