yeah, the good thing about a slow process like a backpiece is you adjust over time, as opposed to being hit all at once, like you're experiencing. again, own that shiiiiiit man!!
I would first take a breath before planning the next piece. easier said than done, but try to absorb the excitement about your current piece before hatching your next....
but when you do, I'd say fill out your arms.
but there aren't any rules for that.