Swing weight is simple, but it's a very antiquated measure. As you've noted - 10g of extra weight in the group throws swing weight off a lot but it is very negligible in the swing.
You can use it to make sure your clubs are similar in feel in relation to each other. If, for example, dry fit your 8i to D0 and your 9i to E6, there's probably something wrong.
If you build your club the way you think you should and it feels good, don't worry about what the swing weight actually is. Just make sure the other clubs are in an appropriate range for what you're trying to achieve.
Or quit messing with this shit and go single length 😃 then you can swing weight them all the same!