Designed a print booklet for my client and I charge an hourly rate. I work for them freelance on an ongoing basis and they already paid that invoice. Next, they got the web designer to use the booklet design as the basis for their new website - in fact they took the PDF of the booklet and used it as inspo for a scrolling-style website. I don't do coding or web design, so I wasn't able to put it together for them, but because the web designer used my PDF and Indesign file to copy the branding, it would have cut hours off his work.
I feel that I should be charging an additional fee for my design being used on the website, even though I technically didn't put in the hours for it. The web designer required my design files in order to make the website. It's just a bit different to supplying a logo, the entire format of the booklet was copied. I was thinking a percentage amount off the original booklet invoice. Or, just an amount I think is "fair", which I have no idea what would be for this situation.
Just wanted to add this is a relatively small family business, always generous and never question things like payment/rates and I know they would be fully understanding, I just need to be professional about it and know how to charge for this kind of thing.
How much should I charge? Where would I get this amount from? How would I bring this up with the client?