The RSGB calculator has been designed to be fairly easy to use and can be found on their website. It is in a MS Excel format, (spreadsheet), but will run on the free 'Open Office' suite.
I'll give you the current link but be warned that this could change with any updates :-
They also have a video of how to use this spreadsheet calculator on the RSGB YouTube site here :-
I hope that helps.
All the best,