Calculating income generated from interest is simple. You'd just include it in interest income in your straw man. Calls, however, are nearly impossible to predict with any degree of certainty. One of mine, for example, has been in "refunding" status for over a year and yet to be called. No worries there. I continue to collect my 5% in the meantime.
So, you need to allow some space to avoid too much income when unexpected cap gains could jeopardize the Medicare thing. In my experience, bond issuer gives 30-45 days redemption notice. Your individual bond carries a call date at which time it may or may not be called prior to its maturity date.
I don't believe the issuer can call prior to call date, so that would give you a clue as to when NOT to expect a call. (I'm not positive about this. You'd want to discuss with bond broker.) If I'm correct about this, you may wish to buy bonds with longer call dates while you're concerned about exceeding income limits for Medicare.
Of course, longer bonds generally carry more risk.
Liquidity is also an issue to consider.
Now that I'm writing this, I realize this is too complex a subject for a message board.
I'd suggest you do some in depth research. Before jumping into munis, be sure you really understand them.
You might also want to look at a muni bond fund. Vanguard offers several, including state specific ones. These allow significant diversification as well as a more predictable income stream.
Hope this helps a little. Good luck.