Hi Paul,
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 :-
https://rsgb.services/public/publications/emc/emf-calculator-v0.1.2-rsgb9e.xlsx
They also have a video of how to use this spreadsheet calculator on the RSGB YouTube site here :-
https://www.youtube.com/watch?v=z9NYlQwbCZQ&t=7s
I hope that helps.
All the best,
Victor