You prolly know, but for the record, spawn vis is a tool that shows where the opponent can spawn based on where you are as they click to respawn or the 5 sec run out.
The spawn system is made so that the z coordinate will almost never count, unless someone makes a very tall vertical map like a sky scraper, where spawns are actually farther away vertically than in any other component. In all ql maps currently available, the answer to your question is that it does not matter.
To add more detail, all was figured out thanks to this post.
As it is explained, the distance is compute per axis, and the largest one is taken. In all maps, the largest axis distance is either x or y, and never z.
This took 1h to make. Donations would require loads of time over many days, considering setting it up, answering questions, deciding on how to use the money, and actually deliver it in a way that makes sense. I seriously considered doing it, but I really cannot afford it now.