In the US, there is no equation. Everything counts but to varying levels. The only guide is that the more difficult an item is for the SA/store to sell, the more they will appreciate you buying it. However, this key item/category likely differs from store to store, SA to SA, and even week to week. If your SA just had another client drop $40k on a Diamond bracelet yesterday but have sold zero silks so far this month, then a 3 cashmere scarf purchase might inspire them to check the back room for something special for you. Or if the same Lindy 34 has been sitting in the display case for two years and you finally come along and buy it, the SM might feel more wiling to approve your Birkin request on the spot as well. IMO, it’s not worth it to try to game the system in this way, just buy the items that you want and can afford and the bag offers will follow.