Magnet and compass through the floor (or walls) alignment trick
A magnet and compass is very useful for working out this sort of alignment. As long as the compass is near enough to the magnet, it will point at the magnet. The distance range is only limited by the size of the magnet.
Note that this magnet is polarized with North-South on the large faces, not the ends like you would expect.
I placed the magnet where the drain hole was going to be, with North/South facing up/down.
Of course, nails in the floor or wall could throw this out a bit, but not by a lot. If the compass was homing in on a nail, you could tell that it changes direction very quickly as it moves, as in, whatever it's pointing at is very near. Though for the nail to become passively magnetized by the magnet it would have to be very near to the magnet to begin with, so it would not throw things out by that much.
Then put the drain pan in place. there's no plumbing attaching to the spout. The idea is that if the drain pan were to fill with water from a leaking washing machine, we'd rather have it drain onto the basement floor than spill over onto the floors on the main floor.
I previously used this trick when I installed a boot nook