what can I from my box?

I have 60x60cm box with 130cm of plant grow space. Running now hps 250w aircooled so reflector is at less than 90f . Growing in soil. 81F when on 70F off. Mylar walls 12 litre pots. foliar feed in veg. 4 plants marketed at 600g per sq meter but wont grow more than 4 plants. made over 2 oz under 250 cfl with plants only 45 cm high. What is the limit i should strive to achive from it considering space and number of plants as the only limit? is 8oz realy possible? That will be third grow.
 
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