Any suggestions on how to calculate whether I should refi my primary? All my calculations on my investment properties, in terms of financing are pretty straight forward. I either buy, rehab and refi into the longest term loan available for that property or I buy and hold long term right out of the gate. All of my projections are based on cashflow, ROI, COC.

With rates as low as they are on Fannie loans, I'm debating whether to refi my primary.  I'm 8 years into a 30 year at 3.875%.  I make extra payments (a few hundred $ extra per month).  If I look on a 30 year amortization schedule my extra payments have placed me, currently, at the 10 year mark in terms of balance.  Looks like if I continue at this rate it will turn my mortgage into a 24 year mortgage which would mean I have 16 years left?  I'm debating refinancing into a 15 year.  At what rate would it make sense?  For some reason I'm having a difficult time calculating because currently, since I'm technically 10 years in, more of my normal payment is paying down principal and then the extra payment is going 100% to principal.  If I do a 15, even though it's shorter I'm starting over.  The 30 also gives me the flexibility of paying the exact amount (no extra payment) which I'm guessing would probably be lower than the 15.  What are actual recent rates anyone has gotten? (I don't mean advertised rates online).  Anyone have a simple way to do this calculation?