If you can open a spreadsheet program similar to Excel, then you can use the following equations to calculate the OG and FG, respectively, where C8 is the initial Brix reading, D8 is the final Brix reading, and $E$3 is the wort correction factor (mine = 0.99 for my refractometer, yours might be anything from 0.95 to 1.05).

=(C8/$E$3)/(258.6-(((C8/$E$3)/258.2)*227.1))+1

=0.00001335 * C8/$E$3 * C8/$E$3 - 0.00003239 * C8/$E$3 * D8/$E$3 + 0.00002916 * D8/$E$3 * D8/$E$3 - 0.002421 * C8/$E$3 + 0.006219 * D8/$E$3 + 1

As far as I can tell, these are the same Petr Novotny formulae used by BrewersFriend.com.

For MUCH more information including great tips from yours truly at top of page 2, see the following link:

https://www.homebrewersassociation.org/forum/index.php?topic=28544.0