You can put a small stainless steel colander (upside down) on the bottom of you kettle during boiling. That will help to protect the bag from scorching. They make small half-round ones that work well.
The auto-siphon is a good idea in place of a ball valve. It may be a bit slower, but not by...
Totally agree with you guys about adjusting the batch size.
Just wanted to make it very simple for a new brewer to order an all-grain recipe kit and do their first BIAB easily.
My first BIAB was with a painter strainer bag too!
Here's an explanation on how to use the calculator.
https://www.homebrewtalk.com/water-requirements-biab-brewing.html
It talks a bit about calculating the boil-off rate too:
For the mashout - after the mash is complete you leave the grains in the kettle, raise the temperature to 170 F, then let it sit for 10 minutes. Remove the grains after that.
For the mashout - after the mash is complete you leave the grains in the kettle, raise the temperature to 170 F, then let it sit for 10 minutes. Remove the grains after that.