2D10 to 2D6

Converting Target Numbers from MechWarrior Third Edition and Classic BattleTech RPG is a non-trivial task, but isn't too hard, and something that I can explain fairly quickly and provide a means for computing what a TN in one system is in the other.  The basic principle here is that any given TN has a certain probability associated with it.  The goal is to match that probability in the 2D10 system to the closest probability match we can get in the 2D6 system.  To illustrate, the probability of rolling any TN on a 2D10 is:


TN P(TN)
2 100.00%
3 99.00%
4 97.00%
5 94.00%
6 90.00%
7 85.00%
8 79.00%
9 72.00%
10 64.00%
11 55.00%
12 45.00%
13 36.00%
14 28.00%
15 21.00%
16 15.00%
17 10.00%
18 6.00%
19 3.00%
20 1.00%

The TN on 2D6 is


TN P(TN)
2 100.00%
3 97.22%
4 91.67%
5 83.33%
6 72.22%
7 58.33%
8 41.67%
9 27.78%
10 16.67%
11 8.33%
12 2.78%

Now, obviously, 2D10 has higher resolution by virtue of having more possible rolls, so we can expect to see several 2D10 results mapping to the same 2D6 result.  Looking for the closest possible matches, we get:

P(2D10)	2D10 TN	2D6 TN	Variance
100.00%	2	2	0.00%
99.00%	3	2	-1.00%
97.00%	4	3	-0.22%
94.00%	5	3	-3.22%
90.00%	6	4	-1.67%
85.00%	7	5	1.67%
79.00%	8	5	-4.33%
72.00%	9	6	-0.22%
64.00%	10	7	5.67%
55.00%	11	7	-3.33%
45.00%	12	8	3.33%
36.00%	13	8	-5.67%
28.00%	14	9	0.22%
21.00%	15	10	4.33%
15.00%	16	10	-1.67%
10.00%	17	11	1.67%
6.00%	18	11	-2.33%
3.00%	19	12	0.22%
1.00%	20	12	-1.78%

The Variance column shows how far the probability of rolling the 2D6 target number is from the 2D10 target number; as you can see from the chart, the difference is less than 5%, except in the case of 10 and 13 on 2D10, which are off by 5.67%. Obviously these conversions are not perfect, but they do provide a means of converting the old 2D10 TNs in material written for the older systems to A Time of War in an approximate fashion that will satisfy any requirement I've ever encountered as a GM.