I'm gonna throw my official welcome in here too Barb. I've used your book since day one and my copy is pretty dog-eared and worn by now.
I want to add to this thread a warning, some mills use an older method of weighing that comes from the weight of so many yards of yarn on a roll before weaving and has nothing to do with what the finished fabric is.
As to fabric weight on Non-Traditional Kilts I use a 9oz, 10 oz, and 11oz in Polyester/Cotton and 100% Cotton.
With these fabrics I'm not as concerned with the weight as it is listed on the bolt, but more with the 'hand'.
Hand is defined as the feel of a fabric and comes from it's weave more than it's thread weight.
Worsted Wool used for Traditional Kilts has a 'hard but loose' hand. The Poly/Cotton I use has a 'hard and tight hand. Denim for Blue Jeans is 'medium soft and loose' I need to have the tightness to the weave due to the nature of the cotton fibers. Without it the kilt soon looks un-kempt. Anyone with a denim kilt will tell you after two washings it is too loose to hold it's shape and starts to bunch up around all the stitching.
My rule of thumb when buying fabric is;
I want a hard, tight weave that feels slightly stiff in my hand. The thickness I like is between .011" and .021" And if I grab and handfull of it and let it hang three feet, the weight of the fabric should almost pull the wrinkles out.