Fresh Air To The Back of FCU's - Which Fresh Air Load Option Is Best?

I am working on an office fitout project trying to establish the cooling loads using CIBSE Gain. The space is cooled by FCU's and we have primary fresh air delivered into the space at 18deg C close to the back of the FCU in the ceiling void. The ceiling void is being used as a return plenum. I have selected the 'conditioned primary fresh air' option and specified the temperature at 18deg C. Is this the best option to use for this scenario? In the manual this option appears to be the best suited rather than the other three options. Has anyone else experienced this?