Longer genes will have more counts, so they will have a higher proportion of mapped reads. A better measure is transcripts per million (TPM), which is essentially what you want but normalized by gene length.

I have no idea how your data is formatted, but this gist is one example of how to get from counts to TPM

I'm not sure what exactly you want, but I assume you don't know how to fix a field using '$' sign. If so, check this cite. https://www.excel-easy.com/functions/cell-references.html

But excel is not efficient to handle a larger dataset. Use R or something similar. They are much better!