Someone can Please let me know where suppose to be the RIGHT position,for the back foot-straps insert(Surfboard) ?
Thank You.

Where your comfortable riding your your board. Every board is different and every rider has their own preference. I'm short and my back foot is at the front of the back pad. I ride strapless mostly though and just know where my most comfortable spot is.

I have put my back foot strap in line with the two front fins, I have done this because when I windsurfed in the waves it was best for turning when the fins was under your foot. When wanting to go fast I would move the fin back.
So for wave sailing I would suggest line it up with your 2 fins that are furthest forward and then see how you feel, it also has to do with your height, weight etc