To determine if a client can contribute more than the $20,500 limit for 403b’s in 2008 – how do you calculate the 15 year service rule that allows an additional contribution of $3000 (total of $23,500).

I understand he has to have 15 service years (client has 27 with the district he is in now) – to figure out if he has averaged less than $5,000 per year in contributions – am I allowed to use the 27 years of service he has currently – or am I only allowed to use the last 15 years.

Can someone help me with this calculation?