Hi Adam,
Thanks for the chat earlier today on this stuff... to build on our discussion
Client EIRP = TxPower + Antenna Gain - Loss
Really comes down (as we chatted about) to getting an idea from the prospect which client(s) they will field and going from there...
They may be with an integrated antenna or a short cable depending if the client is under the bus drivers dashboard, or in a compartment, or in plain site (say on the front windshield of the bus).
Let me know when you get the client info and I will help you from there.