Zekky said:
The bigger the gap, the higher the proc rate to getting a skill up.
From what I remember about crafting, the ideal skill up area is 3-5 levels above your current level.
If I remember right, Synthesis Support also makes no difference to the actual level of your craft, it merely increases your success rate a similar amount as to having X amount of levels (Basic 1, Advanced 3. Something like that anyway).
I didn't take crafting all too seriously, so I could be wrong.