USGBC provides additional guidance

The document doesn't clarify the use of Janitorial/ Mop sinks very well. We have a project that is tracking just barely above 40%, but how we treat the janitor sinks could drop us a hair below that. Four points (including an RP bonus point) are on the line for us.

There's only a brief mention of janitor sinks in the reference guide (it shows up in Table 1 on pg. 103, grouped with Kitchen faucets, and is mentioned peripherally in the implementation section on pg. 102). Perplexingly, there's no "janitor sink" pull-down in the LEED Online template (there was in v2.2), so I'm thinking we just use the Kitchen tab? Then for usage, we were thinking we would have the owner estimate the number of days the maintenance staff comes to clean, how many times they'd fill a bucket during a typical shift, and how big the bucket is (assume 80% of capacity used, to account for cleanser concentrate and the fact that you can't move a bucket that's filled to the brim without it spilling everywhere). Does this approach seem sound?

Your approach for calculating the janitor sink usage is reasonable, but it seems you don't need to include janitor's sinks in the calculations. The NC, Schools, and CS requirements state that calculations include “water closets, urinals, lavatory faucets, showers, kitchen sink faucets, and pre-rinse spray valves.” As there is no drop-down and they aren't included in the list above, not including them in the calculations should be ok.

USGBC does reference that some faucets with low flow rates are not appropriate, especially where the volume of water is predetermined (p. 102 of ID+C 2009). For best practice, you can maximize the efficiency of the janitor's sink with a high efficiency faucet or a foot pedal-operated faucet.