You could also use a payment factor for different interest rates… This will get you in the ballpark…

For 6% interestrate use a factor of 6.0 per $1000 being financed. ie Loan of $100,000( 100 x 6.0= $600) for Principal & Interest on a 30 Yr Fixed.

For 6.25 %use 6.16 as the factor.
For 6.5% use 6.33 as the factor.
For 6.75% use 6.49 as the factor.
For 7.00% use 6.66 as the factor.
For 7.25% use 6.83 as the factor.

This should get you real close to figuring a paymebt but remember that this is just for Principal & interest. Does not include any Taxes & Insurance that may be needed to be added to the payment, Nor does it factor in any PMI.
Hope that helps.