Hi Darryl,
I lived and worked in Fort Worth a good many years ago, and devoted my entire backyard to gardening. A lot of the things I grew (zinnias for flowers -- they were a hobby even that long ago) and sweetcorn and watermelons for veggies) seemed to actually enjoy the heat. As you know, in Texas, for many days in a row, you can exceed 100 degrees. In fact, the local weathermen refused to call it "hot" unless the temperature exceeded 100 degrees. I did have a big garden hose, one inch in diameter, and I used a lot of water during the Summer. And I had a Bermuda grass lawn that took a lot of water also.
"I want to make a small shade tunnel in a full sun area of my yard, but I can't find any info on which percentage shade cloth is best for seedlings."
If you have a tent, you might try erecting it in your garden with the sides open in the up position.
ZM