Interest Only Amortizing w/ Higher Payment

I haven't had any luck with online mortgage calculators trying to figure the payoff date with a fixed payment.
160,000 loan balance @ 6.375% 10 year interest only, converts to 20 year am.
Total payment now is $998.15mo. I want to pay the same $1250mo every month.
So obviously the interest will go down every month while the principal portion of the payment goes up.
All the calculators I've seen online ask "how much additional payment do you want to make, rather than a fixed payment.

Thanks in advance,

Seems like it should be paid off in 18 yrs, assuming fixed payment of 1250 from month 1 on and a fixed interest rate of 6.375%.