I'm not sure about recommended weights, but if you are just adding a stiff spring to the stock shock I would consider this...the stock shock has very little damping to slow compression/rebound down. If you put a really stiff spring on it, it will rebound back pretty fast like a pogo stick.
If you are planning on ridding very aggressively on rough terrain I would suggest buying a better shock rather than wasting your $ on a spring.