Maybe the AngelList data is why startups have trouble finding good engineers? There is no good way of finding out what a good developer should get, and that is because the companies (including yours) like it that way.

I only have anecdotal data, I don't think many people have more, but I am very sure a decent senior dev can get way more than what you are offering.

I think there is more to a job than just the salary, but at least be open about the fact that there are open positions doing similar work in the same city offering significantly more.