Where is the source of the Tay?

It is widely acknowledged that the source of the Tay river system is high up in a corrie on Ben Lui, at the top of the Cononish. But where exactly? There seem to be two watercourses competing, with a spectacular if less likely third option as well.

This is the Allt Coire Laoigh (Lui), the most dominant watercourse at the top of the Cononish, and logically, the source will be at the top of this.

However, there is another claim, see below........