/ic/ - Artwork/Critique

How do into 1 point perspective? I'm trying to recreate something similar to pic related pic related, but facing straight into the lava, not at an angle. The problem is, I'm having trouble making the platforms emerge from the lava at the same height. How do the platforms' heights change in relation to their proximity? Let's say the vanishing point is at 0 on the y-axis, and the nearest point to us is -10. If a platform at (0,-10) is 5 pixels tall, should a platform at (0,-2) be 1 pixel tall, or is it more complicated than that?