AGA uses 4 ranks per stone, BGA uses 7 ranks per stone, I use 6 ranks per stone in my youth club.
I feel that 4.6 ranks per stone is better than 4 ranks per stone, but perhaps the ratio needs to be higher still. (also see 2021 Rating and rank adjustments - #528 by gennan)
It is related to the question of which rank you assign to a raw novice. I (3d EGF) can typically give 8 stones handicap on 9x9 to a raw novice 8 year old, to who I assign a rank of ~42k, i.e. a gap of about ~44 ranks.
With 4.6 ranks per stone, 8 handicap would correspond to a gap of ~33 ranks, so a typical raw novice 8 year old would then have a rank of ~30k. I think that somewhat overestimates their level, but it’s not so easy to verify this by measuring actual rank gaps with handicap games on 19x19.