By the Association of Photographers, which one can use to help determine what the fee would be for the ‘Additional use’ of one’s work.
So if you start by letting the client know what the fee would be for ‘Standard use’ first – which would be the amount that you would want to be paid for the use of your work in either 2 media for 1 year or 1 media for 2 years, in 1 country, i.e. your Base Usage Rate (BUR) – then should they want you to provide them with more than that, you can use this Usage Calculator here…
.. to help you determine what the additional (+%) fee would be, for that amount of ‘Additional use’ of your work.
Very useful – so a big thanks to the Association of Photographers for this.