Scale objects marginally!

Heya,

I want to scale objects marginally. I am scaling an object while dragging at runtime, everything runs fine, but when I drag objects it scales all over the place, like very speed. I want to drag at certain speed or sensitivity so to speak, it should be like only scale, when I drag my mouse to any screenposition(left, right, up, down) at certain level/value to either snap back to localScale when less than dragvalue on either side or scale it when we drag more than the specified position. I don't know if that it would be clear enough, so I am providing graph. Here:

I would be very helpful if someone could provide just a pseudo code for achieving this.

Thanks.

pyrateN