FDM: Clear=transparent part I intend and would like to print a transparent hemisphere. I have a Taulman T-Glase clear 1.75 mm filament and I have XTC-3D.

You can read this interesting page, Hacking t-glase to look more like glass!

First question, what kind of printing settings should I use? Should I go for a low infill percentage or a high one? Should I go for line or hexagon? I would say 5% line but perhaps there is a better setting.

Then, how should I use XTC-3D as mentioned on the above link? I'm a little bit confused how it can make the part more transparent.
First I would hedge your expectations.. I am very familiar with t-glase. It is very translucent but it is not clear and no amount of post treatment will make it 100% clear. Their example is the only exception where you have a simple print where it has only a single layer. By applying the clear coating they have normalized the wall and made the surface even. Thus no more light diffusion. 

You could likely get the same effect with good sanding but good luck sanding a 1 wall thick print. Also note they do not show any solid parts. If they cannot get a solid part to look good and clear, then really you shouldn't expect to be able to. 

Any infill will cause it to be non clear. Any extra shells will also create air pockets, lines, etc and make it non clear. If you really want clear, FDM is not the way to go. The only way I could imagine to print an item clear with FDM is to create a cast, then cast it with resin or glass.. 

If you consider how the plastic is applied, you can see there is no real way make it 100% air pocket free.. See below,



Image taken from here. 

The best I can recommend for settings. Make your temp spot on. If you boil the material you will have less translucency. Play with over extruding (too much material) in a Limited amount. Might reduce gaps. I would also try to print in BIG THICK lines. Less lines, less diffusions.