How should advanced chapters be calculated? (Patreon)

My chapters are extremely long for those of a webnovel, especially on Royal Road, and I've been advised by other authors that I should make sure that I'm charging appropriately for them. I do want it to be fair, but I'm fine with it being cheap since I'm not in dire need of the money. What does "fair" mean to you guys? Do keep in mind that my average chapter length is something like 3.5k words, with the longest of my chapters being almost 6k words long, which is triple what many authors on RR aim for.