"I've made this twice. The first time it didn't thicken properly and I had to throw it out. The second time I cooked it for an extra two minutes and it still didn't thicken. I turned up the burner to medium heat and cooked it another 5-6 minutes and then it thickened. I think the water in your double boiler has to be more than a simmer. Looking for color change or to "coat the back of the spoon" isn't enough. You need to notice a significant thickening.

As far as the number of lemons, you have to adjust for size. I used three large lemons and had over half a cup of juice.

In the end, it came out great."