When I build an anchor off of trees I only use a cordolette, 3 locking carabiners and 100 ft of 9.5 static with a static strength of 5,700 + lbs. It's simple, redundant, equalized, no extension and quick to set up. It uses minimal gear and in the end way lass expensive. There has not been an instance where I needed anything else. Please I beg you to hire a guide for a day, it will be well worth it. Nobody should get hurt Tope roping !
Thanks ! But of course my question was really for securing myself while setting my TR. Normally to build my anchor I use a Quad made of 7mm static cord. Pretty bombproof.
But the way you stated the question leaves probably a few people concerned. I'll break it down more than I probably should;
Would a Beal Rando 8mm 20m should be long and secure enough to secure myself while setting TR Anchor/ rap down from the top to set up my Anchor ?
#1 In terms of strength a Beal Rando is undoubtedly strong enough to hold your static weight while setting TR's. You weren't really questioning that right?
#2 In terms of length how could we possibly answer other than saying "Yes, as long as you only need 20m".
#3 In terms of "secure enough" well that really depends on how you are using it doesn't it? The attachment to an anchor. Your attachment to the rope. Knot on a bight? Clove-hitch? Friction hitch? ATC? Gri-Gri?
The methods of which someone uses a edge safety line are varied and run the spectrum in terms of "current practice" vs. "what we did 30 years ago"...
So to sum up your question was stated in a way where some might suggest you seek out some instruction. A focused one day course on top-rope building from a qualified guide could very well be a day well spent.