I've just had to make a Y rod for my split elevators. I taped the two pusrods together at each end then bent them together, it means I now have two 100% identical rods. To find where to mount the servo, I moved the elevator pushrod by hand, then moved it form side to side, while checking the throws. When I found the exact spot where there was no diff, I marked it and mounted the servo so that the arm was perfectly in line with the mark.