Joshua - It is a big unknown to me what percentage of the forum readers, besides the person requesting information, are interested in the same question. Also unknown is what level of detail is appropriate to answer a question. I believe that it is generally prudent to avoid the situation known as, " I just wanted to know the time, not how your watch works."
People are comfortable when the same amount of heat generated is the amount lost. Your body is similar to an automobile in that 75% of the energy used results in heat and only about 25% in useful work. We need to know the activity level in order to calculate how much insulation is required for a specific temperature. The base metabolic rate (1 MET) is the amount of energy you are burning when laying down resting but awake. When asleep you generate about .8 this amount (.8 MET), doing camp chores twice this amount (2.5 MET) and backpacking an average of about seven times this amount (7 MET).
As an ultralight backpacker, you want to select, from your inventory, the lightest combination of insulation that will insure comfort for all activities within the estimated temperature extremes. You first determine how much insulation you need to sleep comfortably at some specified minimum temperature. In Doug's case, he was planning on using a bag that had been tested using the EN13537 procedure. I believe this is the most accurate testing system available. His Marmot Hydrogen's 30F EN13537 Lower Comfort rating means that it averages 5.88 clo. The insulation required for a temperature rating can be found on page 32 in EN13537 Convert m2 K/W insulation values to clo values by dividing by .155.
The BPL reviews on most clothing items give the insulation weight in oz/yd2. The clo/oz ratings for the most common backpacking used insulations are:
Cotton .04 clo/oz
Merino wool .08 clo/oz
Polartec 100,200, and 300 series .16 clo/oz
Polarguard 3D, .63 clo/oz
Exceloft .68 clo/oz
Polarguard Delta, .68 clo/oz.
Climashield HL, .68 clo/oz.
Down (550 fill) .70
Primaloft Sport, .74 clo/oz.
Climashield Combat, .79 clo/oz.
Climashield XP, .82 clo/oz.
Primaloft One, .84 clo/oz.
Down (850+ fill) 2.53 clo/oz
Multiply the insulation weight in oz/yd2 times the clo/oz to determine the Item clo value. This tells you how warm it would be if it covered your complete body. You then need to multiply this number by the % of the body is covered by the item to calculate the clo Iclu value. The clo Iclu value allows you to determine the effective insulation by combining items with different insulation values. The average vest covers 36% and the average jacket covers 48% of your total body area. These % adjustments were made to the appropriate clothing items. After adding all of the clo Iclu values, convert this number back to m2K/W by multiply by .155. Then look at the above referenced graph to determine what temperature that combination yields for comfortable sleep.
For activities other than sleep, you generate more heat and so your insulation needs are reduced in proportion to your MET level. If we wanted comfortable sleep at 40F, we can see from the above referenced chart that the insulation required would be approximately .76 m2K/W. We divide by ~.155 to find the equivalent clo value of 4.84. We then need to divide this by increased heat generated from camp activities or 2.5/0.8 which yields the required insulation of 1.55 Iclu clo that is shown in the chart. The clothing combination that is most appropriate for this scenario is determined by combining items to find the closest clo Iclu total. In this case it happened to be our base clothing + Patagonia Micro Puff + Montbell Thermawrap + rain gear yielding 1.95 clo Iclu.
The information about how good the proprietary Exceloft insulation is illustrates an important point: to keep things simple, just focus on the thickness of the combined insulation layers.