How much of the saddle rail is suiteable to use Is it possible to safely set the seat to the max to get the saddle as far forward as it will go. Or should it only be clamped within the marked range 0 to 3.5? I have marked the points in red. The reason I ask is because I don't want the saddle to tilt forward while riding if I have it set to max. !enter image description here

As long as no part of the clamp touches the rails outside the max lines, you should be safe.

The scale is simply there to help you. It would be nice if it ran right up to the max lines, but it might still be helpful depending on the size/design of your clamp.