Turbo tax
I too struggled to find an explanation for Excess Cost Basis. However, after reviewing several expert sites, there were the best explanations I could find:
https://www.google.com/amp/s/www.fool.com/millionacres/amp/taxes/1031-exchanges/1031-exchange-calcul...
https://tax-lawyer-texas.com/2018/05/28/calculation-of-basis-in-new-property-in-a-section-1031-like-...
Hope these help.
