Help calculating stock investment returns vs S&P 500

I do a bit of stock investing and am interested in comparing the individual return of a single stock vs the S&P 500. I have created a spreadsheet that works by comparing to an equivalent portfolio invested in the S&P. The spreadsheet works if you only buy currently. If you sell, you begin to collect residual shares of the equivalent S&P 500 portfolio (either too few or too many shares) which makes the calculation incorrect and does not provide for a fair comparison.

I have attached a sample to this post.

Please take a look at the example and give me your thoughts. Is there a better way to approach this? Is it possible to accommodate an equivalent portfolio based on the value of the transaction?

I greatly appreciate any assistance you could provide.