I'm a lazy gardener. If nature will do the work for me, I say let it. I inherited a few small greenhouses from my grandfather, so I use them mostly for peppers and cotton in the summer, and getting a jump on spring. My method may not work for other people as, like I said, I
make let nature do most of my work for me.
I'm also highly miserly when it comes to the garden. I use what I have to hand. Some years it's raggedy old cotton sheets draped over the West side of the greenhouse (the hottest sun is from the west-south-west and the west (late afternoon-evening) here - where your hottest part of the day may differ). Usually, it's the leftover cover from the carrots. That's about 5 foot wide - because that's how wide it came, but you can get different widths - and as long as I needed it. Again, I just drape this over the greenhouse trying to cover the area that gets the hottest sun. Basically I don't measure because I'm not buying or cutting something new. Also, I don't cover the whole greenhouse, just the hottest sides.
My observations with my greenhouse is that it's not so much the temperature that does it for me but the lack of airflow. Those 8x10s I'm using would be far more useful if they had a door each end so that I could open and close the doors to suit the weather. As it is, the air stagnates and can contribute to mildew and other problems. With a fan, the air moves about and the
water in the soil and in the plants evaporates and allows the plants to cool.
Another thing I'm experimenting with is using a light coloured mulch in the greenhouse so that the soil doesn't heat up as much in the summer.