You can estimate the body surface area on an adult that has been burned by using multiples of 9.
An adult who has been burned, the percent of the body involved can be calculated as follows:
- Head = 9%
- Chest (front) = 9%
- Abdomen (front) = 9%
- Upper/mid/low back and buttocks = 18%
- Each arm = 9%
- Each palm = 1%
- Groin = 1%
- Each leg = 18% total (front = 9%, back = 9%)
As an example, if both legs (18% x 2 = 36%), the groin (1%) and the front chest and abdomen were burned, this would involve 55% of the body.
So what I did is create a table of these percentages in Excel (or you could use any spreadsheet software) and then multiplied the armor class bonus (either for ascending or descending AC systems) by the fraction of coverage.
Here is an example that I popped into numbers (the mac excel analog):
Area Fraction AC Bonus AC Fraction
Head 0.1 2 0.2
Chest, Abdomen, Back, Buttocks 0.36 6 2.16
Right Arm 0.09 2 0.18
Left Arm 0.09 2 0.18
Right Leg 0.18 2 0.36
Left Leg 0.18 2 0.36
Total AC Bonus 3.44
The total AC bonus for this character wearing leather on his whole body(AC Bonus +2), but having a splint mail breastplate giving AC bonus of +6 on his chest would have a total AC bonus of 3 (use typical rounding rules i.e. > 3.5 = 4). If this character found even a steel helmet, or increased his legs to steel plate, it would kick him up to AC bonus 4.
I always liked this, since I have always had access to spreadsheet software. It takes 5 minutes to put into excel, and it keeps the single AC system active but adds in the ability to customize the PC's armor class in infinite ways. This can also be fun if you find a single +2 steel helmet and you can see how that affects the total AC.
*for those of you who want to create the above table - what I did is multiply each fraction by the armor class bonus, and then summed these values to create the total*