I think nothing is better than starting with pen and paper and writing down every bit of non-routine maintenance you can think of that you might need and how often and what it cost last time etc. It won’t take long before you are well on the way to answering the question.
I would also aim to have a fair bit of ‘contingency’ funding which you aim to have as a kind of basal residue in the fund that you could use if several of the expensive things ended up costing (say) 50% more than your reasonable estimates or there was something big you didn’t think of.