/ic/ - Artwork/Critique

Im learning to draw perspective grids.
Can someone please explain to me or give me some pointers about how to transfer lenght and mesurements from the horizontal plane to the vertical plane in 2 point perspective?
If I understand correctly, using the diagonal vanishing points you can make a rectangle with all equal sides ( aka a fucking square), then from there you can use dividing and multiplying to make the grid.
But How do I draw a cube with all equal sides?
How do I transfer length from horizontal lines to vertical lines?
Only way I though of was using the horizontal vanishing points.
also tell me everything wrong about my pic

Thank you anons