No one in the Philippines has come up with a real answer to this, but basically, I played nine cantrips, three colors and only 16 land at the Manila Prerelease and finished 4-1-1 without getting color screwed.
I think there must be some formula where you can remove land, like an old rule of thumb where 2 Impulse = -1 land.
Maybe 1 land for every 3 or every 4 cantrips, with considerations for color?
I would say remove one land for every 4 cantrips
I played a Draft - not sealed, but draft - deck in which I played 17 land. I also ran 3 Opts, 2 Probes and Fact or Fiction. I chose not to consider the Probes or FoF cantrips, but I did count the opts as such. Of course, I could always dump extra land to the Probes, but I like to run 18 lands typically, and I went less this time due to the Opts.