I think it's possible to get a 1220 working length out of 1500, I'm not sure my design will be good enough to do it though. My reasoning went something like this: 1500 - 1220 = 280. That means that I can have a maximum of 280mm from the back of the Y-axis bearing to the centre of the spindle assuming there is nothing else blocking the movement and the front Y bearing is not ahead of the spindle centre line. That's certainly achievable but... with this frame design the legs will, I think, limit my maximum X travel to 100mm less than the rail length which would mean the gantry would have to be no more 180mm and that feels too narrow to me. To be fair I could probably manage it with 1600mm on X but I figured if I'm going to make it larger I might as well make it comfortably large :-)