I wish I had an exact answer to your question. My teacher just took a small fist sized hunk of the MMF in her hand and put the Gum Tex in it and worked it in and then added more until it felt right. She said that it needed more than the Wilton to feel right. My suggestiion would be to invest in a package of wilton and do it with that first, so you see what it is supposed to feel like and then add the gum tex to the MMF until it feels the same.
gum-tex or gum tragacanth is the stuff they put into chewing gum to make it chewee n not swallowable (dang did i explain that right?) But it makes it so that the fondant is more pliable while it is still uncured and once it has cured it is harder. You can roll out gumpaste or gumpaste amended fondant thinner than regular fondant. that is also why they recommend that you let your mixture rest for a while to let the gum hydrate (like making bread dough, have to let it rest for the gluten to activate) then kneading it aligns the strands of gum making it more elastic and stronger and less likely to crack n pull apart on you.
Man I hope that I explained it so that you could understand what I was tryin to say. 
LOL
I usually add the gumtex to my mmf with the sugar I usually add about 3 tbs for every bag or 4lbs. of powdered sugar (that would be 1/4 cup). It makes the fondant nice n useable for making flowers.
But I still prefer using plain ol' MMF for covering cakes. It depends on what I am using the fondant for.
If I am making figures I like using the "wilton" fondant just because it is already mixed to a decent smooth consistency (no one is gonna eat them things anyway) but for flowers I like adding the gum-tex.
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