Modeling Office Space in an Existing High Rise Building

We're using eQuest to model a tenant improvement office space, which is about half of one story in a 28 story high rise office building in LA. We're using ASHRAE 90.1-2010 modeling protocol (LEED v4 commercial interiors project, ID+C). Our basic strategy is: 1. Model the entire high rise building using floor multipliers and include base building plant equipment (building chilled water service and per-floor air handlers). 2. Apportion the project space energy use from the total by area, per the LEED reference guide. For this we will exclude the top and bottom story energy use, presumably by putting a meter on each floor and subtracting them from total before the apportioning calculations. My question is: Can we take credit (model proposed different than baseline) for efficiency measures of the base building (efficient chilled water service, efficient air handlers, high performance envelope, etc)? Also, please let me know if our strategy is way off base. It seems more complicated than it needs to be, so I'm looking for a sanity check before we dig in to a high rise to get our little T.I. certified. Is there a guidance document or precedent for this strategy anywhere? I've conducted a somewhat thorough search, but maybe I missed something. Any insight or guidance would be appreciated.