I have made and installed several gates with this problem. The formular is. Hinge centres x lift divided by length divided by two.The top hinge is fitted to the gate between the posts. The bottom hinge is fitted to the inside of the gate post and a L shaped bracket on the inside of the gate. I used 1/2 aluminium ball joints obtained from a general bearing supplier. The beauty of this system is the ball joints don't have to swivel as much and the gate is perfectly horizontal In both the closed and open positions. I used hydraulic arm automations but can be manual as well. hope this is of help to all.
