When I started making long bows I took some advice from Mark Horn. He was using .040 back and .050 belly on some u-finish bow blanks he was selling. Since then I have seen other bowyers, including the wtt guys and they do the opposite. But they (wtt) are mostly after speed than durability I suppose and some of them thin their glass down considerably from what I can tell. I trap the belly when I do it. Belly is more narow than the back. Seems you loose less draw weight this way and still acheive light tips when doing it. If you intend to loose five pounds or more then trapping the back may be the ticket. I've never done that because I want all the weight I can keep. Black widow traps the back but I'm sure they figure that into their stack. As far as the glass goes, I don't know what is best. There is a good bowyer by the name of Bob Sarrels that uses the thinner glass on the belly and his shoot very well. I may try it some day. I think the important thing for durability is to keep the glass at 20 to 25 percent of the total stack.