My new client had an installment sale in July 2010, she managed to dig up the 2010 tax return..the list of forms in the front shows that they charged her for the 6252, but of course there is NONE included in the copy of the tax return!

I did find the original amortization schedule for the installment sale, but it shows that payments began in Jan 2011, do I really need her to find the 2011 return? 

Or since I have the amortization schedule and the gross profit % on the 2018 return, can I just fudge in the basis on the top of the 6252 to make it match the gross profit %..shes got the principal and interest amounts received for 2019.