I've got Nitro Planes Zlin 50's too, and having a tough time getting it right. I've flown it, but it's a handful to say the least! It's got a Saito 56, still have the servos in the rear, battery mounted as far forward as I can get it. With that setup, the CG comes out at about 110mm from the leading edge. I've flown it that way. It flies fairly well until you start to get aggressive with the inputs, then things get really wild. For instance, pull up into a tight loop, and it will just pancake or snap out of it. Same thing with a tight bank and yank turn. It's like the wing blocks the elevator causing it to lose effect. When landing, you have to come in hot, for it'll fall off the wing in a heartbeat. I just finished mounting 14oz of lead to the front edge of engine mount. That brought the CG forward to about 90mm. I plan to try that tomorrow or the next day and I'll let you know if that improves it. If it does, I'll move the servos like you did.