Bill, this approach has never been published by any source that I am aware of. None the less, it is based on science rather than marketing hype. It should satisfy your requirement for something quick and simple.
1) Most quilt insulation materials are close enough in insulating value that you can just estimate the warmth by the thickness. The weight is a different matter but you already understand that and your decision to go with PG Delta and Quantum is sound. Determine the thickness from Mike Martin’s table at http://www.backpackinglight.com/cgi-bin/backpackinglight/bpl_sleeping_bag_position_statement.html.
2) All temperature rating charts, including Mikes, are based on some assumed theoretical average person’s metabolism. The most common is defined by ISO 8996, Ver. 2004. It is a male, 20 yrs old, 154.3 lbs, and 66.9 inches tall. This person will generate heat based on their daily basal metabolic rate of about 1,740.3 C. The table ratings also generally assume the person is wearing about .5 clo of clothing in the bag. This is equivalent to long underwear or pajamas.
3. Enter your sex, age, weight, and height into the calculator at http://www.bmi-calculator.net/bmr-calculator/.
3) If you generate less BMR heat than the theoretical average person, you need more insulation and conversely. Divide the theoretical average person’s BMR by yours and then multiply Mike’s thickness number by the result.
Let me give you an example. I am a 63 year old male, I weigh 180 lbs, and my height is 5’ 11”. My BMR is 1475.6 using the calculator. Let’s assume I want a quilt that will keep me warm to 20 degree F and not wear any insulation in the bag other than my long underwear. Mike Martin’s table shows a thickness requirement of 2.2 inches for the theoretical average person. 1740.3/1475.6 *2.2 = approx 2.6 inches is what I would need to stay comfortable at 20F. By contrast to me, if young Ryan F. was making a new quilt for his upcoming trip to the Sierras, he would probably be fine, in his long underwear, with close to 1/2" less insulation than what I would need.
If you wear your Cocoon insulated top and bottom to bed, you should subtract that insulation loft from the required quilt loft.
Also note that Mike’s table as well as other rating system tables generally assumes you are protected from the wind, your bag isn’t wet, you have an adequate sleeping pad, and you haven’t degraded your loft by overstuffing.