The full details on how Medicare Part B premiums are calculated can be found on this page at Medicare.gov. The income brackets for the calculation can be found on that website. For example, those who file a “married filing jointly” return with a modified adjusted gross income two years ago of $176,000 or less will pay $148.50 per month.
In short, the premium is based on the modified adjusted gross income (MAGI) reported on your IRS tax return from 2 years ago. However, MaxiFi doesn't know what your MAGI is from two years ago, so it uses your current-year MAGI as a surrogate for both your previous two years. You can adjust for any discrepancy between what MaxiFi calculates and what you expect to pay for Medicare Part B premiums for those first two years with either special expenses or special receipts.
After the first two years, MaxiFi will have your earnings history etc. and be able to calculate your Medicare Part B premium based on actual MAGI.
MaxiFi does not consider Medicare Part D, so if you wish to account for that ongoing expense, you’d need to enter it as a series of special expenses.
Note that you can influence the calculation of these premiums for future years in Settings & Assumptions / Misc. If you want to assume that future premiums will merely keep pace with inflation, set this assumption about real rate of return to 0%. If you want to build a more conservative or cautious model, you could assume that the real growth rate is, for example, 3% or, in other words, that these premiums will rise 3% over and above your assumed inflation rate.