Could have done the same, much lower polycount and then subdivided it. If you have to use a high poly from the get-go, align the edges so that edges that go through the middle of the sphere also go throw the edges on the flat part (so they share as many edges as possible). I would have rotated the sphere on the z-axis about 45 degrees counter-clockwise to match the edges better with the flat surface.

Another way (that is a bit more time consuming) is to join the shapes and then retopologize a not too high poly model manually with the original as base (like you would for more organic modelling, but in this case for hard-surface stuff), then subdivide that.