Noob crafting questions about quality increase. !!

[NoRyoKu]

Hi

Noobie crafting question, I have Culinarian at 50 and starting to learn crafting and how to get max quality. I have noticed that when I use "Touch" skills, the quality
doesn't always increase by the same value, one hit can increase for 10% and the other like 35%

My questions are :
-Is there a time to use those touch action for better increase? Is it when the quality is normal / good / excellent?
- Should I wait and use observe to get to the desired quality before using touch?, Should I use synthesis skills to change quality and then use touch action when I have the quality I want?

ALSO, Which stats is making the touch abilities to give you more quality on each step, is it craftmanship?

Thx!