A New Spin on Masteries
A Rhugandy house rule

Background:
Some of my players had a long running and stubborn criticism of the Hero Wars rules. The complaint was that there isn't much of a difference between an ability of 20 and 10W (not counting the improved chance of a critical). If a character with a 10W ability rolls a 15, he will loose to a character with a 20 ability rating who rolled a 14. The following house rule was accepted by my players as a reasonable fix for this situation, and we have used it for some time now to the satisfaction of all. It does add a bit of math to the game, but my players were willing to make that trade-off. 

How does it work? 
One mastery = target number of 20. 
Subtract your ability rating (without the masteries) from the roll. 
A result of zero or less is a critical.
A natural 20 is a failure. 
Each extra mastery after the first "bumps" the result. 

Examples: 
A character with a 6W1 ability rolls a 10. The adjusted result is a 4 (10 - 6), a success.
A character with a 6W1 ability rolls a 5. The adjusted result is -1 (5 - 6), a critical.
A character with a 6W2 ability rolls a 10. The adjusted result is a 4 (10 - 6), a success that bumps to a critical.
A character with a 6W2 ability rolls a 20. This is a failure that bumps to a success.