Help with Proper level for a CP Vitesse Saddle

[fastandpro]

I am seting up the fit a bit better on my Giant ocr road bike and have to get the saddle level. The Vitesse is only level at the nose then curves up at the back. Ajustment is no problem I just need to know were to measure to get the level correct. ie on the nose, in the nose and the v at the back, or at the nose and high part on the sides,

One other alinement tip worth the effort
I also placed a long stright edge down the middle and alligned the centre line with the top tube it looked ok but was out quite a bit.
Thanks
James