Holy crow your brave, I stay further back from semi's and what not, at least 40 ft. away and I find that dangerously close!
I am not sure how much benefit I am losing by staying back so far but I am sure not going to go any closer to find out.. I do notice the draft effect if I change lanes I start to lose speed. Give that a try and you will find your gains or as it may be losses in mph. That's bout all I suggest, I am sure somebody has figured out the actual gains made by drafting at different distances but its not me.
10-20ft. sounds too close to react to a sudden brake lock up from the drafted vehicle, 40ft. may be too close
as well..