Deore Derailleur w/ Tiagra STI Shifter Compatibility
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.


That's true, except for Tiagra 4700, which rather inexplicably uses the 11 speed pull ratio. 

See https://bicycles.stackexchange.com/a/52277/24228


  Presently there are two different 10-speed Tiagra generations, 4600 and 4700. (So the STI from one is ST-4600 and the other is ST-4700, etc). 4600 uses the same classic actuation ratio that all pre-11-speed Shimano road groups used other than first generation Dura-Ace. 4700, however, is an oddball group. It's 10-speed but uses the new 11-speed actuation ratio