i wouldn't build it as you have drawn it, there's a design flaw in your drawing.think about apllying force ont eh platform and think what the reaction to the force is going to be.. force has to be canceled out.. the strap on the platform is too low , it's no so much a problem when you on the edge of the platform as the force will act as a lever and pull the wire into the tree, the rest of the force will be dissapated through the ladder..

however where is the force going when you stand close to the tree ?...there would not be a big lever and such the force would push the platform down the tree instead of pulling the string into the tree.

the most easiest solution for this is where the strap attaches to the platform , put a vertical riser on teh platform in essence making a bigger lever when you stand at the tree.. then also secure the platform at the platform height or maybe a tad lower..

If you look at factory made tree stand you will see that in almost all cases the top of the ladder where the strap attaches to the tree is about 2 feet above the standing platform..