Since I started actively refereeing Swords & Wizardry several years ago I come up with some handy concrete guidelines that help make up a ruling without staddling the game with overly complicated mechanics.
First off I look at Swords & Wizardry and the classic editions to see what tools I can come with.
- There is the to hit roll.
- Saving Throws
- Some type of roll based on a characteristic.
- A skill roll.
This is best used when the ruling involves throwning, swinging, or hitting something. If it is just about hitting the target with no real damage I usually set the target number based off AC 10 (or 9 depending on the edition). The same with targeting a piece of floor or a specific section of a wall. If just hitting a large building, wall, rock, etc. Then I will add +2. I will also use the range guidelines for darts and other thrown weapons as a guide to when to impose range modifiers.
I use saving throw to resolve actions where player is attempting to have his character do something non-lethal to another character. A character wants to do disarm or trip an opponent. I will generally say he needs to hit the target and the target gets a save. If the save fails then the character is successful in his action.
The implication is that it harder to do certain things against higher level character or monsters. I feel this is OK as in my mind higher level or HD represent characters/monsters with more points or build in other systems.
The benchmark I go by here is that it is -4 to hit an invisible opponent or to fight in complete darkness. Generally this means modifiers range from +4 to -4.
There is also the issues with modifiers granted by characteristics. Some editions like ADnD have extensive modifiers while other don't. I found I was happy with this chart.
Some type of roll based on a characteristic.
I don't use this a lot myself but other referees. The basic options are
1) Roll under the characteristic with a d20
2) Roll a d20 add the characteristic and get 20 or higher
3) Multiply the characteristic by 5 and roll under the number with percentile dice.
A skill roll.
With the introduction of the Thief class skill rolls became part of DnD. The original class used percentile dice modified by race and dexterity. This is something I never really liked. I preferred something similar to 3rd Edition version which is a d20 roll and beat a target number. I consider skill rolls valuable because they allow the creation of character class that are better at various non-combat things. Ultimately what I adopted was roll a d20 and equal or beat a 15 modified by the relevant characteristic and any bonus given to you by your class.
However the spirit of the oldest editions make for a game where characters can attempt anything. So instead of skill I added abilities. That way anybody can still try to pick a lock but burglars are better than anybody else.
By combining these various elements I can come with the mechanics needed to adjudicate just about any a players wants to do in my campaigns. With the virtue of still making the game feel like you are playing a classic edition.