I mash my grains in a 5 gallon paint strainer bag. Works great. I have a strainer that fits over the top of my kettle so I can put the bag in the strainer and let it drain and/or rinse it/sparge it.
For conversion from grain to extract of vice versa, a good rule of thumb is 1 lb of grain = 2/3 lb DME or 3/4 lb LME.
Do you have to do anything with you mash set up to keep the temperature stable? How much do you sparge with? What temperature do you sparge?