The most efficient way to convert ADC to percentage is to run the numbers in a spreadsheet and get the "trendline" which is nothing more than the exact equation that will yield your result with exact precision every time (well close enough). Since you do not want to deal with decimal points you can just do the math and save the answer in an unsigned int or unsigned char type variable and it will truncate anything beyond the decimal. Your result will be within +/- 1%.
Here is a simple sheet I did in Google Sheets that demonstrates this:https://docs.google.com/spreadsheets/d/1VAMKumpeXOC4dFI3i60GayT2NixQBTJ25uusbiFy-gA/pubhtml
The main take away from this sheet is that your magic number is 0.098.
So (ADC number * 0.098) = your percentage***or***
(percentage / 0.098) = your ADC value