i would like to help but i have a few questions for ya, what type of fan is cooling your mh? and how strong are those fans in the side wall of your grow box, from the looks it seems you have adequit air exchange, what are the temps out side your box? is the box in an enclosed room or a room that gets plenty aeration? air exchange may not be your problem you may just be sucking that hot air back into the box

as for your nutes i think it would be safe to say you can start them at 300-400 ppms and gradually work your way up, i go up to 1500 by the 3rd week of veg and hit as high as 1800-1900 in flower, i think as soon as you get your temps in check you will have less evaporation cause those plants being that small are not using a gallon a day, also do you know your humidity?