Distance of downwind leg from runway in glider landing pattern?
I was taught (by various publications and compenent CFIs) that angles are a better thing to focus on than distances.  For example, this publication suggests:


  You  should  turn  from  your  crosswind  leg  (or  from  your  45  entry)  onto  your downwind leg when the runway centerline, or an extension of it, is 25-30° below the horizon. Using angles automatically compensates for variations in altitude.


For me, this makes sense for several reasons:


Judging angles is much easier than judging distances.
The AGL pattern altitude might differ depending on the airport/field you are landing at.
If you are landing out, you should ALWAYS fly a normal pattern if possible.  This may force you to fly an abbreviated pattern closer to the ground.


Obvously, adjustments to this will need to be made for a crosswind.