I started donating regularly but following the thought process:
Some amount of money exists which is small enough that I wouldn't notice not having it.
This is clearly a lower bound on how much I am morally obligated to donate, because not having it costs me 0 utility, but giving it awa generates positive utility for someone else.
I ended up donating £1/month, but committing never to cancel this and periodically review it. I now donate much, much more.
Compare the benefits of encouraging other people to take a similar approach with the potentially harm associated with this approach going wrong, specifically moral licensing kicking in at relatively small donation amounts.