jjfoo,
I presume you include atmospheric humidity, even though you don't mention it specifically.
yea, probably the biggest factor!
If you have tons of holes or smartpots (fabric pots) you are going to lose more water and need lower ec. If you have a R.H. of 30 you will need different nutes than a guy with 80.
I have no holes at all in my buckets for root pruining or gas exchange, so I don't get near the evap as people that drill holes in their buckets.
Here is an interesting read:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC367454/pdf/plntphys00221-0149.pdf