Five months later and back from the dead for your first post? Nice!
Whether the “formula is right” or not as I am not sure about the context, “what about the 8% return?” nonsense (or numbers just like it) comes up time and time again. What I said earlier on an MSD thread:
MSDs are usually the way to go but I know many people that have good reasons not to always use them.
One is if you plan on a potential lease transfer at some point it will narrow the number of people that can cough it up. Another depends on if you are a 1099/IC or if you own your company and how you structure your taxes/deductions as to actual expenses or mileage and how much you need for those deductions.
MSDs also need to be looked at along with what other debts and/or obligations you may have such as student loans, college or private school expenses for kids, paying for elderly parents, second homes or investment properties, etc. Obviously paying down other debts depending on the interest rate, etc. and having capital available for other opportunities or emergencies will make a difference depending on your personal financial situation.
Compounding interest (both ways), taxes and opportunity costs cannot be overlooked.
