my final production code is caculating the scale factor using the real cropping rectangle, and applying this to the RelativeCroptRect.
Then it works perfect.

 protected override void ResizeCropRect(Point location)
        {
            base.ResizeCropRect(location);
            if (CropRatio > float.Epsilon)
            {
                var r = this.GetCropRectangle();
                float h = RelativeCropRect.Height * r.Width / this.CropRatio / r.Height;
                this.RelativeCropRect = new RectangleF(this.RelativeCropRect.X, this.RelativeCropRect.Y, this.RelativeCropRect.Width, h);
            }
        }

Ok, it works . Thanks

    public class MyImageEditor : RadImageEditor
    {

        private float _CropRatio = 0F;
        public float CropRatio
        {
            get { return _CropRatio; }
            set
            {
                _CropRatio = value; ((MyCanvasElement)ImageEditorElement.CanvasElement).CropRatio = _CropRatio;
            }
        }

        protected override RadImageEditorElement CreateImageEditorElement()
        {
            MyImageEditorelement element = new MyImageEditorelement();
            return element;
        }

        public override string ThemeClassName
        {
            get
            {
                return typeof(RadImageEditor).FullName;
            }
        }
    }
    public class MyImageEditorelement : RadImageEditorElement
    {

        protected override Type ThemeEffectiveType
        {
            get
            {
                return typeof(MyImageEditorelement);
            }
        }
        protected override ImageEditorCanvasElement CreateCanvasElement()
        {
            return new MyCanvasElement(this);
        }
    }
    public class MyCanvasElement : ImageEditorCanvasElement
    {

        public float CropRatio { get; set; }
        public MyCanvasElement(MyImageEditorelement imageEditorElement) : base(imageEditorElement)
        {
            if (CropRatio==0F) CropRatio = 0.5F;
        }

        protected override void ResizeCropRect(Point location)
        {
            base.ResizeCropRect(location);
            float h = this.RelativeCropRect.Height;
            if (CropRatio>float.Epsilon)
                h = this.RelativeCropRect.Width / this.CropRatio;
            this.RelativeCropRect = new RectangleF(this.RelativeCropRect.X, this.RelativeCropRect.Y, this.RelativeCropRect.Width, h);
        }
    }