Randy (DreamHammock owner behind the scenes) recently added a calculator to his site. He doesn't claim it's perfect, but it should get you to a very close estimate:
I plugged in your numbers and came up with the following (assuming a 60 in width):
1.0 ripstop: 23 oz
1.8 tafetta: 28 oz
I have a 1.8 night grid tafetta on order right now and can't wait until it gets here!