I could have overlooked something and would be happy to stand corrected.  But how is that a circular calculation when it's a simple with and without computation?

• Medical expenses only matters if I/D is to be taken, in which case, there will be no above-the-line deduction for $300.
• Medical expenses wouldn't matter at all if the above-the-line deduction for $300 is to be taken.

There are various convoluted calculations in the Code but it is inconceivable that Congress would inject such complexity into a basic deduction that is meant to benefit more taxpayers.

There may be limited circumstances where it would be more beneficial to claim I/D even if it results in a higher taxable income (and in all likelihood, the difference should just be marginal where such choices even exist) but that should be very rare, if at all.