How to measure the distance to horizon line one-point perspective?
This solution will work any shape of room you have. Cubic, rectangular (as I demonstrate), and even rooms you aren't looking at from straight on (in which case you rotate the top elevation, a common perspective technique).

Click on any image for a larger view.


Layout the general lines. 


a) You need three horizontal lines: your horizon line, a ground line (somewhere below the horizon), and a vantage line. In this example, my vantage line is below the ground line. The vantage line represents the distance of the viewer from the horizon.  
b) You also need to choose a vanishing point somewhere on the horizon line.
c) You can pick a vantage point, but in this case its only purpose is to establish where to put vantage line
d) Draw the front elevation of your room, with the scale dimensions. The front elevation needs to rest its bottom-most line (or point, if rotated for 2-point perspective) on the Ground Line
e) Draw the top elevation of your room, with the scale dimensions. The top elevation needs to be under the Vantage Line, with its top-most line (or point, if rotated for 2-point perspective) on the Vantage Line.  
f) Draw the front view of your room, aligned with both the front and top elevations


The real trick to this technique is mapping the angles between corners to the Measuring Points on the Horizon Line.


a) From a bottom corner of your top elevation, draw a line going through the opposite corner (such as bottom-left to top-right) and continue that line all the way to the Horizon Line
b) Where those lines intersect, mark that point as a Measuring Point
c) Repeat for the other bottom corner


Draw your normal perspective lines on your room


a) Don't worry about the back wall quite yet, just make sure your perspective lines go either all the way to the vanishing point, or far enough that you can be reasonably sure you have room for your back wall


Now the technique comes into play. You'll use the Measuring Points to map a projection to tell you where the back wall begins


a) From a bottom corner of your perspective room, draw a line to the matching Measuring Point. So, if you pick the bottom-right corner, draw a line towards the Measuring Point that connects to the bottom-right corner of your top elevation. 
b) Where this new line intersects your normal floor perspective lines represents the corner of your back wall.
c) You don't need to map the other bottom corner, unless you're drawing a room/shape that doesn't have parallel front and back walls
d) Draw a horizontal line from the back corner you measured to the other side of the floor


Standard perspective techniques will give you the top of the back wall.


a) Draw vertical lines from the bottom corners, and the points where they intercept are your ceiling's back corners


Go over your final guidelines and clean it up!  


a) You're done!