"Should I put the taxes I paid on the "Purchase Commissions or Fees Paid" line, or calculate how many shares roughly equal the taxes I paid in cash?"
"None of the above" is the correct answer here. The vesting of the RSU creates compensation and compensation requires withholding, simple as that. If you pay the withholding from cash in pocket, your cash in pocket goes down but your shares you receive goes up. If you pay the withholding by selling shares then your cash in pocket stays intact but your shares you receive goes down. The taxes you paid here have everything to do with compensation and nothing to do with your sale of the stock. The sale of the stock is a completely different matter.
Leave the shares "for taxes" box blank and you did not pay any purchase commissions or fees to acquire the stock.