1. Look at the Faraday.
2. Look at the wiki. There is the optical layout in PNG and PDF.
3. 5% (0.8mm) and 2.5%(0.4mm) sounds a big difference for the difficulty, but if you say so, it is not so different.
Actualy, if you can get to the 5% level, it is easy to get to the 1-2% level as I did last time.
The problem is we are at the 15-20% level and can not improve it.