Almost perfect lemon meringue pie

I made a lemon meringue pie for the first time over the weekend. It looks nice, doesn't it? The crust was deliciously crisp and flaky, and the meringue layer was lovely. The only problem was that the lemon custard didn't set. It tasted great, but a runny custard doesn't make for a very pretty slice of pie. The recipe was from The Gourmet Cookbook - in my experience, the recipes in that book are flawless, and I followed the instructions to the letter, so I'm not sure what went wrong here.

I'm looking for some help from you. Do you have any tips for making lemon meringue pie? A foolproof lemon custard recipe? I'm not going to give the full pie recipe that I used here, but I'll summarize how I made the custard. Let me know if the proportions or cooking times don't look right to you. My plan is to try the pie again with the same crust and meringue, but use the custard recipe from The New Best Recipe by Cook's Illustrated. Once I find the perfect recipe, I'll let you know!

*Update*
The perfect recipe can be found here.

Here's how I made the custard:

4 large egg yolks
1 cup sugar
4 1/2 tbs cornstarch
1/4 tsp salt
1 cup water
1/2 cup whole milk
1 tbs unsalted butter
2 tsp finely grated lemon zest
1/2 cup fresh lemon juice

Whisk together egg yolks. Whisk together sugar, cornstarch, and salt in a saucepan. Add water and milk, whisk until cornstarch dissolves. Bring to a boil, whisk until mixture thickens. Slowly whisk 1 cup milk mixture into yolks, then whisk yolk mixture into milk mixture. Simmer, whisking, for 3 minutes. Remove from heat and whisk in butter, zest, and lemon juice until smooth. Pour filling into warm pie shell, top with meringue, bake until meringue is golden, about 15 minutes.