I do have some experience in play testing miniature games (not legion), and while there are a number of ways to initially come up with point costs, generally speaking it starts with something like deciding on an arbitrary number of points for a standard game. You want the number to be large enough so that you have granularity in adjusting point costs, but small enough for the numbers not to be intimidating. Then you come up with a point cost for your baseline unit/miniature. So for example if you are going for your basic grunt to cost about 1% of the total points for a standard game a 1,000 point game would have basic grunts be 10 points. Then you go from there for other units. Maybe a commander costs about 10 times that of a grunt, an elite trooper 50% more, and a total bad a** 20 times more. These are just your initial estimates that will very likely change during playtesting. Sometimes you adjust point costs, and sometimes you adjust stats/abilities.
Originally Legion was going to be played at 1,000 points (again from interviews or live streams. I don't remember exactly where), but during playtesting it was found that a game was taking too long. So the point level of a game was dropped to by 20%. However I don't think it is a coincidence that the game was originally designed for 1,000 points, and that a rebel trooper costs 10. I would not be surprised that if early on in the design of Legion stormtroopers also cost 10 points each, but were found to be a little stronger than rebels so got a 10% point increase. A strict formula for how much a unit will cost is usually not a good way to balance a game. Some abilities will synergies better or worse together effecting the strength of a unit that will not show up in a points formula. Also how well a unit will synergies within its own faction should effect the price. Two units identical on paper can have vastly different power levels depending on what else is available to a specific army. These are some of the considerations that need to be taken into account when pricing a unit that a formula will not show.
Again, I have/had no part in playtesting Legion. So this is all just speculation on my part based somewhat on my experience as a playtester for other systems. Coming up with a points formula for units can be fun, and useful for initial ideas for homebrew units, but it is not likely how existing units received their point cost. But if it is something that you enjoy doing, I say keep at it! I do find it interesting what you are coming up with. I thought I'd just share a small piece of how the sausage of game design is made.