>>7685076
Station point. There will always be a point in your drawing where angles measured with a protractor are the same as angles in perspective. You can use that to find vanishing points for any angle, and draw your new cube.

I rotated it 15* just so the vanishing points stay on the page, but same concept.

If you want to measure out the sides to be the same, look up the 'measuring line'. But honestly, just guess, it's good enough most of the time.