Tactical Mastery
Overview
Tactical offence rating determines how much damage your tactical attacks deal. It can be improved by increasing one of your statistics (based on your class) and by direct bonuses on gear. The exact amount of tactical offence given per point of the appropriate statistic also depends on your class. Tactical offence rating is converted into a percentage before it is used to calculate your damage. The formula for this is similar to the one used for Critical Rating.
 % = (Tactical Offence Rating  Base Rating) / (300 * Your Level + (Tactical Offence Rating  Base Rating)) + Base %
Base %  Max %  Base Rating  Max Rating 

0%  20%  0 * Your Level  75 * Your Level 
20%  40%  75 * Your Level  150 * Your Level 
40%  60%  150 * Your Level  225 * Your Level 
60%  80%  225 * Your Level  300 * Your Level 
80%  180%  300 * Your Level 
In order to calculate your tactical offence %, find the row in the above table for which your tactical offence rating is between the row's base rating and max rating. For example, given a level 95 character with 37,111 tactical offence rating, you would use the last row (base rating = 300 * 95 = 28,500, no max rating). Then your tactical offence % would be:
 (37,111  300 * 95) / (300 * 95 + (37,111  300 * 95)) + 80% = 103.2%
You can calculate how much tactical offence rating you need to reach a given tactical offence % in a similar fashion. First pick the row for which the desired tactical offence % is between the base % and the max %. Then, use the following formula:
 Tactical Offence Rating = (300 * Your Level * (Tactical Offence %  Base %)) / (1  (Tactical Offence %  Base %)) + Base Rating
Again, using a level 95 character as an example, if you wanted a tactical offence % of 75%, we would use the fourth row. Then, the needed tactical offence rating would be:
 (300 * 95 * (0.75  0.60)) / (1  (0.75  0.60)) + 225 * 95 = 26458
Level differences impose a penalty if the target is sufficiently lower level than you. This penalty ranges from 0% to 5%, and is calculated using the following formula:
 Tactical Offense Penalty = RANGE(0, (Your Level  Target Level)  1, 5)%
For example, taking the level 95 character with 103.2% tactical offence from before, if they were attacking a level 90 target, then this would impose a 4% ((95  90)  1) penalty on the character's tactical offence %, leaving them with an effective 99.2% tactical offence.
Tactical Offence from Statistic by Class
Different classes receive different amounts of tactical offence rating per point of statistic.

Development History
Tactical offence rating was added with the Siege of Mirkwood expansion.