How low to install a ball valve on a boil kettle?

Assuming you are installing a tube on the inside of the kettle/valve, you do not want it to be too high to the point where the tube has a significant gap between the end of the tube and the bottom of the kettle. Also, depending on your internal fitting, you want to ensure you have enough clearance from the bottom on the inside to account for your washer and even your tools to attach the fittings on the inside.

I would say the most important thing is a well-drilled hole. Scrappy, poorly-drilled holes make it difficult to fit the component parts in and can make it possible for bacteria to collect between brews.

Often you'll be hooking some kind of hose to the outside of your valve, so gravity will do the work. I think for tightness-of-seal you're right - you shouldn't install it below the top of the 'roll' at the bottom. Good luck! Measure twice, cut once!

edit

Based on your comment, I think the answer to your question is 'no'. Placement really depends on your setup, and in fact I've seen boil kettles with the output valve drilled in the dead center on the bottom of the kettle. This was being used in a homemade tiered system, and while it wouldn't be for everyone, it fit this person's brewing environment and setup perfectly and worked a treat. So, I guess my original advice stands - measure twice, cut once! How will your kettle be used? What environment will it be used in? Is that environment likely to change?