Synth quality exp bonus is based off the difference between the initial quality and the ending quality and is in no way affected by the inital quality.
Evidence:
Target Synth - Rosewood Lumber(46)
Target Crafter - CRP 43
Item Used - Rosewood Log +3
For the first synth, I didn't record the inital quality, but here's the end result.
I was baffled by the miniscule bonus for such high quality, so I tried another one, this time deliberately trying to get as little quality as possible.
Initial Quality: 311
ARGH! I was so frustrated after seeing that. I purposely was waiting to use by Rosewood Log +3s until after 1.20 patch because I thought I'd get more exp...
If you're not convinced by just these two synths, here's one other data point that seals the deal. I made a Peiste Leather(41) on my LTW 41 using an NQ Peiste Skin. This was before I did the Rosewood tests, so I didn't note the starting quality, but the ending quality was 31. I got a 10% exp bonus.