VTA Cartridge

Tonearm tube must be parallel to the record surface when the needle is on the record. This is the starting point. 

chakster  yes but my tonearm tube is conical, so no way to use this option, thank's

Dear @audiosens : VTA is a set up parameter that normally is made by listening tests, one and again till we are satisfied with what we are hearing in our room/system.

There are no universal rules, you have to listen through VTA tests evaluations using LP's tracks that you really know as your " hand's fingers ".

When VTA is ok the sound is just open with good tonal balance over the frequency ranges and everything is in focus very precise focus.

Normally if VTA tonearm talil is at a little high position the high frequencies are more prominent and the bass range could goes a little on the dull side and when the tonearm tail goes lower than the horizontal line happens the other way around so you only have " to play " with till you are satisfied with.

The only person that can judge about is you.

Regards and enjoy the MUSIC NOT DISTORTIONS,
R.

Your cartridge is attached to a flat plate.  That is what should be parallel to the disk as a starting point. 

Usually, but not always, that plate is parallel to a a linear extension of a line bisecting the upper and lower lines of your conical arm.  You can often see that line by shining a bright flashlight on the arm from the side.

To begin with, based on his description (see above), I think the OP is conflating azimuth with VTA:
"Could I consider that the front face of my cartidge (DV20x2H) is perpendicular (90 degree) relative to the vinyle surface, can we concider that it is a way to have a good VTA ?"
This seems to me to be a reference to azimuth, not VTA at all. Also, as regards VTA, the fact that a tonearm wand is cylindrical is in no way a hindrance for setting VTA. (To the OP:) To set VTA, you view the arm tube from the side and with respect to the surface of an LP sitting on the platter. Viewed from the side, a cylinder will present two straight parallel lines, just as if it were square. If you make those two parallel lines parallel to the LP surface, for most styli, that is a good starting point for setting VTA. But, like Raul says, you tune by ear from that starting point.  Better yet, start by setting the top of the headshell parallel to the LP surface. That's for VTA.