For stereo, vertical alignment matters more than hitting exactly 64mm. A small vertical mismatch makes viewing harder, while the baseline does not need to match average eye spacing exactly.

The most useful improvement is to mount both phones in the same orientation rather than flipping one 180°. That makes it much easier to keep the corresponding lenses at the same height and pointing the same direction.

A practical way to align them is to loosen the bracket, press the fronts of both phones/camera bumps against a straight flat surface (like a table edge), then tighten everything. This helps keep the cameras parallel.

You may also want a wider baseline than 64mm for stronger stereo effect, especially for subjects that are not very close. Only reduce the spacing for close subjects.

If simplicity is the goal, some users even stack both phones in one holder and trigger them with a wireless remote. The slight front-to-back offset is usually negligible with phone cameras because depth of field is very large.

So: prioritize matching orientation, parallel aim, and minimal vertical misalignment; treat baseline as adjustable based on subject distance rather than fixed at 64mm.