Hiyas. This post offers some workarounds for dealing with the two major complaints of using perspective in ZBrush (too much perspective even at the lowest setting and the inability to save the vanishing point with the document).
1. Decreasing perspective beyond the lowest ‘distort’ setting:
Perspective can be decreased beyond the lowest ‘distort’ setting by using ‘Universal Perspective’ (shift-clicking the ‘Persp’ button) and then moving the object back toward the rear clipping plane.
To move the object backward in space, activate the Move Gyro and then drag the mouse/pen upwards outside the gyro. The object will shrink as it recedes in space, decreasing perspective.
After moving the object backward in space, activate the Scale Gyro and scale the object up to the desired size. The change in perspective will still be apparent after scaling.
Here are some examples: