Finally got the hang of my job's flow. So I returned to some of the old code I had laying around. First up is #mypaint 's symmetry guides. For rotational snowflake guides and slanted mirrored guides, first step was making a function that calculated the angle of each line and extended.
Next component will have to be calculating the mirroring when it's sloped. Haven't quite worked out the math for that yet, but it's progress! Yay.