I suppose two schools of thought here.
1/ what’s the going rate (if you know this) i.e. what others of similar experience and ability charge?
2/ salary divided by a working year – then add on factor/s for risk of consultancy vs. salaried, your holidays, NI, your office / home space, equipment, and so on.
* don’t undersell yourself* – even it’s seen as a ‘hobby’ (sort of).. if it takes off, then you’ve made a rod for your own back. better to give an upfront reasonable rate that reflects your worth, even if they decline (or negotiate down). I pitched for a couple of jobs when I first did the same thing, didn’t ask *too much* but still got a knockback. They were clearly looking for someone _very_ cheap, and that’s not a route I wanted to go down..
good luck!