Heat the oven to 350°F and mix the graham crumbs, 1/4 cup sugar and melted butter.
Press the crumbs firmly into the bottom and sides of a 9-inch pie plate and bake for 8 minutes.
Whisk the condensed milk, egg yolks, lemon juice and zest until thick, pour into the crust, and bake for 15 minutes.
Beat the egg whites and cream of tartar to soft peaks, gradually add the remaining 1/2 cup sugar, and beat until glossy.
Spread the meringue over the hot filling to the crust edge and bake for 10 to 12 minutes until browned and 160°F.
Cool for 1 hour, then refrigerate for at least 4 hours before slicing.