Jeff is correct, it should be calculated on the initial term that the tenant leases for, you can decide if you want to pay a commission on any renewals. The commission is usually paid out of the deposit to the agents/brokers first and then the rest goes to the owner. 2.5% commission is very low for a lease, the average lease commission is 6-10%, if you were to offer 6%, this would be offering 3% to the selling agent. You sound like you might also want to get additional information on the contracts and disclosures, this is a legal binding contract and you don't want to make any mistakes as this could be costly to your boss.
Good luck and Have a great day!
Heather Paul, Realtor