How to get a square in perspective?

[Kartoffel]

My first post here, so first of all, hello, everybody!
I will show you some work of mine soon but first I have a question:

How can I get a square in perspective drawing?
I start a drawing by drawing two lines from each vanishing point so I get a rectangular surface. But how do I make these lines of equal length so that i get a square?

[Signature]

I don't know if there's a real easy way but I'd look in Successful Drawing by Loomis.
http://fineart.interweb.cz/LoomisBooks/successful/page_01.htm
It looks like estimated squares ... but I might be wrong.

The complicated way would look like that:
http://www.andyart.de/images/persp002.jpg
You'd only need the lower yellow line in the front view though.

If that doesn't make sense look at the whole "tutorial"!
Link (http://64.191.99.205/~andyt/forum/index.php?showtopic=32)

[Danilo]

Look My perspective tips and tricks tread down. There is tuts created by me, and you can find there something that can help. If you have some questions post replys in my tread.
cu

[Kartoffel]

@Signature: Thank you, that helped me alot!

@Danilo: I had seen yours before but it didn't help me with my problem. The only pictures which explains making squares are the second and third one and I did't get how the distances betwenn the vanishing points and the 45-degree-point are made right.

[twindragon]

I have always hated perspective. especially making things curved and the right proportion because trying to do it the way that signature showed gets complicated when doing huge detailed drawings. I know how to make things like telephone poles the same distance apart at least :D