Technically, the basis used is that immediately before sale. So unless the sale took place at Jan 1 12:00 AM or Dec 31 11:59 PM you can't use the 7203.

Start with the beginning of year basis and add/subtract/prorate somehow through the date of sale. Then divide by 4. Then reduce the amount on the 7203 by this. Do a spreadsheet for the computation.

Repeat each year.

Don't forget to reduce AAA accordingly, too.