Is there a proper way to space crank arms in relation to the bottom bracket? Ive noticed most builds are generally +10mm on the right side in relation to the bottom bracket, but i feel like maybe they should be even for leg ergonomics? Is there a standard here from edge of bb to the outer edge of where the crankarm mounts to the bottom bracket?
The gap you're talking about is more or less random and does not itself affect how symmetrical the Q-factor winds up being.
The quick visual check for Q symmetry is to eyeball against the chainstays, which is close enough in almost all cases to know whether there's a problem unless the frame either has an asymmetrical rear end or is smashed. To get an exact measurement, one uses the depth gauge part of a vernier against either the seat tube of a round-tubed bike or the back rim (after checking dish) for bikes with asymmetrical frames.