Sounds like a high mash like everyone else said. Also, I might throw in my new http://biabcalculator.com tool as a suggestion to help get your water right.
I have just created http://biabcalculator.com to meet the need of a simple calculator specifically for BIAB. It can be used to calculate water needed, strike temperature and even the maximum your brew kettle can hold.
You're headed toward a brown beer if you add either one. .75 isn't much just leave it out. You really won't miss it. Just do your water calculations accordingly.
That's about as low a temperature as you will want to run that yeast. Make sure to raise it to 68 for a day if you can to do some cleanup. Should be a clean fermentation.
Like the others said, it can be done. I have a small pot and wrote up this calculator to help figure out how much water I needed and how much volume the grain bag and water would take up total: http://biabcalculator.com
Thanks for the kind words wilserbrewer! I have some in return - I just received one of your grain bags, hop bags and pulleys and they are excellent, high quality products that were a very good value - so thank you!
If you have any doubts please check out my http://biabcalculator.com tool. I wrote a total mash volume value in there that takes the volume of grain and water into account so you know the minimum volume of the kettle needed for your recipe and brewing parameters.
Just to be clear, trub in the mashing phase refers to the liquid left in your boil kettle along with hops and break material after you complete your normal transfer procedure to the fermenter. Yeast and flocculation and all that are an entirely different phase and have nothing to do with your...
If you use 15 pounds of grain in a 5.5 gallon batch, that grain + water is going to take up about 9 gallons worth of space and once all the water and grain is in your brew kettle for the mash.