I have Shimano Deore RD M591 Rear Derailleur (http://www.jensonusa.com/Shimano-Deore-M591-9-SPD-Rear-Derailleur) that I am having trouble dialing in shifting when used with Tiagra 4700 2x10 STI shifters (http://www.chainreactioncycles.com/us/en/shimano-tiagra-4700-2x10-speed-sti-shifter-set/rp-prod137777) and a SLX 11-36t 10 speed cassette. I was working off of the general hypothesis that a 9 speed mountain derailleur would have the same pull ratio as the 10 speed STI shifters. I can adjust shifting to work as needed for the first 3 highest gears, but then when I shift beyond that towards the "easier" rear cogs it is consistently skipping over a cog (from 3rd to 5th for example). This continues up to the 10th cog, shifting two gears for every one click of the shifter. If I adjust cable tension so shifting is smooth in the middle or low end of the gear range, it is then not accurate at the high end of the gear range.
My goal was to extend my climbing gear range as much as possible so I could take my winter/cyclocross frame and turn it into a gravel bike capable of climbing some steep Vermont dirt roads.
Any thoughts or ideas appreciated. Thanks!