If you are designing an amortization schedule and then want to see how additional money on the principal will affect your payoff, then you need to use (or download) a 30-year mortgage schedule (not a 21-year or 19-year schedule as the calculations wouldn't work correctly using this method) and then go to year 11 to see where you are in your payoff and what you can do to accelerate the payoff if that is your goal. You can download this schedule (http://www.wisebread.com/files/fruganomics/DIY_acceleration.xls), replace $153,000 with $200,000 and replace the 6% with 7%. You'll also need to refigure the payment amount (I should have designed the spreadsheet a bit better; but the formula for you should use is =PMT(7%/12,360,-153000,0,0) which will equal $1017.91 (principal and interest only, I haven't included escrow amounts for property taxes). If you are in year 11, your balance should be between $125K and $128K. So, starting there (say in month 132), you can add principal amounts to the monthly payment. If you made an extra payment of $1000 in month 132, you'd pay the loan off three months early.

Let me know if you have more questions or if you need some validation of the numbers.