People have been happy with .51 cuben. It is definitely strong enough.
I am particularly rough on my gear so I have gone with .74 on both of my cuben shelters.
.74 is half again as heavy as .51, but still much lighter, stronger and less stretch than silnylon.
Z-packs, Mountain Laurel and others will make their gear with .74 if you really feel insecure.
Most agree that .74 is overkill. .51 is still stronger than silnylon. But .51 is not as abrasive resistant, which normally shouldn't be an issue with a tarp, backpack yes, but how much abrasion does a tarp see?