I think you're better off building a spreadsheet for your situation rather than an online calculator. This way you can add as many or as little extra payments (and of various sizes) as you wish as well as save as many scenarios as you can think of. You can have a set of columns for the loan amortized over 30 years with no extra payments and then another set where you alter the principal paid in a given month as much as you want to provide an easy comparison. I'll call the former set of columns 'Planned' and the other set 'Actual'.

I'd put the date, payment number, total payment, amount of escrow, principal, interest, balance, and LTV% as columns for the Planned side and then a reduced version of that on the Actual side plus a 'Extra Pmt Amt' column which will be assumed to go all toward principal.

I'll provide an example spreadsheet with using $624113 as the purchase price @ 4% interest for 30 years and 20% down payment as well as some made up extra payment amounts for the hypothetical first year of the loan but here is how it's built.

Looking forward, you can figure out the principal and interest (and thus, future LTV%) if you made the minimum required payments by using the PMT function. i.e. =pmt(interest rate listed as a whole number/(12 months * 100 to turn percentage into proportion), number of payments, -loan amount) or in your example specifically this would be =pmt(4/(12*100),360,-(whatever your starting loan amount was which is purchase price minus down payment)). This is more of a verification than anything to see that your current portion of your monthly payment devoted to P & I is properly amortized, i.e. is equal to $2384.69. In the interest column, you can find out how much of that $2384.69 will be going toward interest moving forward by using =balance*whole number interest rate/(number of months in year*100 to turn percentage into proportion). Then the principal amount on your next payment will just be $2384.69 minus the interest you just calculated. After that, you can subtract the principal from the balance month-by-month. Thus, the interest amount you calculate each month is going down (since the balance is lower each month).

When doing modeling with various extra payments in column F, keep in mind the loan is still amortized over 30 years so just look when the balance in column I gets to $0.

Cheers,