Length of cord.

I have a Dandy designer,could anybody tell me the correct length of cord that loops to the bed base should be.My problem is that once up its tends to collapse before I get the other side up.With two people no problem as one holds it in place until I attach the sides but I want to be able to use it on my own .I have had gas struts fitted to make the lifting easy.

I know it's a long time but in case anyone is looking for an answer.
If the roof bar drops the cord is too long.

It needs to be sufficiently long to allow the window and door walls to be lifted through the gap between the two roof bars. Probably a gap of about 1 to 2 inches each side would be about right.