Go to a strap hinge. The piano hinge i tried to use on an extension to my workbench was not strong enough and was harder to align that I thought it would be. I also made fold down legs to support the table when folded up. Reinforced the legs with ply so there was room to put 2 hinges on each leg. Made it so the legs fold up out of the way and actually cross beside each other when folded up. The strap hinges were quite easy to attach so they aligned perfectly.
Glued a layer of MDF to the ply for thickness, then screwed a piece of 1/3 inch high density Masonite to the top and waxed the heck out of it so glue won't stick. The extra stiffness is nice and the MDF helps keep it perfectly flat. The extra thickness will also let you use heaver, longer screws to hold it in place. I attached the 16" wide by about 60 inch long extension to the back of a HF workbench. But I did cut some hardwood strips that I attached under the front and back aprons or skirts, of the table. They reinforced where the hinges attach and also provide a much wider edge clamping area than the 3/4 piece that is the apron. Finally, put some 1/4 x 20 threaded height adjusters on the bottom of the fold down legs. Really helps manage work and space to move around in a smallish shop.
The legs are folded up in the first picture, down in the second. In the second you can see how i used the ply to reinforce the leg, and to allow me to put on two hinges, which do not show. To allow the legs to fold up, I offset one leg so it would fit behind the other. The ply supports were cut to a length less than half the length of the fold down top so they wouldn't interfere with each other when folded. Very stable. I marked the floor where the legs go when the top is perfectly flat, but you won't need to do that.