Free cash flow or the share count is not in BXP's XBRL, so there is nothing to calculate.
Work on BXP in a spreadsheet:
Excel workbook (a sheet per section, ten years by column, charts, links to the filings) ·
CSV.
For a Google Sheet that stays up to date, paste =IMPORTDATA("https://tickdelta.com/company/BXP/export.csv") into a cell.