There's not much point in putting mylar at the bottom of the plants because the leaves at the top will block the light making it impossible for the mylar to reflect any back up. The best place you can put the mylar is at the sides of the plants. What you've got to think of also is your headroom, the plants will double or even triple in height by the time they've finished flowering, have you got enough height in your grow-space? What size pots are you using? I use 12 inch pots and when the plants have grown a bit and got bushy, I prune all but the 4 strongest branches on each plant (so I have 4 main bud sites on each plant). If the pots are any smaller you should have less main branches like 2 or 3. Or a better idea would be to transplant into 12 inch pots or bigger so you can have more branches.
You can't really say what the yield will be, there are many factors which affect the yield- genetics, strain, soil quality, nutrients, light etc. so it's impossible to say. What strain are you growing? I grow Afghani. I couldn't say exactly what the yield of my last crop was because it wasn't weighed and I'm no good at eyeing it, but I had a shoebox full of buds from 4 plants. I know this isn't much but it's not bad for only 250w.