I think Henry Shire's site says to allow 5% for shrinkage of Tyvek after washing. A Tyvek tarp has been on my DIY list for a while. I haven't gotten around to making one yet. If I were you, I would buy extra material just in case.
I am thinking that I am going to use Tyvek Tape to reinforce the edges of my tarp when I make it. Be sure to read the sewing instructions listed in the Tyvek article. Sewing can make the HomeWrap Tyvek easy to tear and tarp tie outs have a lot of tension on them. I am leaning toward leaving extra material on the ends on the Tyvek tarp and then tying the guy lines on the tarp with something like a sheet bend knot.
“Light thinks it travels faster than anything but it is wrong. No matter how fast light travels, it finds the darkness has always got there first, and is waiting for it."