Alkbud, your description of your grow room is very poor, using centimeters is not needed unless your a gnome.

How many 'plants' do you intend to grow with only a 250w HPS??

You can set your plants desired hight by growing it to your desired hight, although remember this, the plant does its best growing in the start of flowering, therefore grow past your desire so you should minimize it by about 3-4 inchs.

I can see you growing 1 or 2 plants under the 250w HPS, but I dont see more than 5-6.

The reflecter is simple enough, you can use any kind of home appliance you have, you dont simply need a glass mirror thats curved. You can use simply a piece of cardboard with boards on the back to keep er steady and mylar it. You could also bend the cardboard if you want to put it around the top of your light.