The whole process is way faster because I don't have to determine the size of the background layer of multi layer skins

Each skins came with 2 type of file (in the AssetBundles/painting folder):
- texture files that end with "_tex" ("bulisituoer_3_tex" and "bulisituoer_3_rw_tex" for that bristol skin)
- skin data file "bulisituoer_3" (for that bristol skin)
The skin data file contain the correct size for the background layer and to find it you have to:
-open the skin data file with AssetStudio
-go to asset list and find the only asset called "Transform#" (and not "RectTransform#")
-go to the preview tab and find the pathID for it "m_father" ( for example: https://i.imgur.com/5hljr1n.png )
- find the only RectTransform# asset with that pathID
- go to the preview tab and grab the correct x and y values (for example: https://i.imgur.com/KvIacYb.png )