Here is a quick mock-up of a finished spell chart.

Making these charts takes some time, there are over 5,000 possible ways to fill in the first column, not to mention the other eight. Yikes!

How does it work? Wizard rolls backgammom betting dice + 3d6 when casting a spell. Go down to the result and over to the column representing the spell level.

Purple is critical success, Green is success, Red is failure, Black is critical failure.

Other fluff information: Wizards actually use these charts, Arcanists study the charts for other reasons, but cannot actually use them to cast spells.

A quadrant represents a specific range of numbers. The primary quadrant is 5-10. The second quadrant is 11-20, the third is 21-30, etc. The final quadrant is 67-82.

Various magic items or wizard abilities grant automatic spell success within a quadrant.

Vocabulary note: In the final version, the word will be each of these sections will be heptant (one piece of a seven part whole) instead of quadrant (one piece of a four part whole).

https://drive.google.com/file/d/0B8faGW02LDlSOTduVWRJdGVuaUk/view?usp=sharing

    <img class="alignnone" src="https://www.sycarion.com/wp-content/uploads/2017/10/WIP.png" alt="" />

Here's one of the charts so far. Spell level runs along the top, b3d6 roll result along the left side. (Remember b is the backgammon betting die)

Purple = critical success
Green = success
Orange = failure
Black = critical failure

In an OSR game, INT bonuses affect the result by allowing the player to take the result of the roll or a number of rows down equal to the INT bonus.

So if a character has +1 INT, the player can take the result of the roll or the result one row down from the roll. If they roll an 82, they can take either the result for 82 or for 5.

    I'm working on the chart for b3d6, but wanted to throw this out there while it's rattling around in my brain.

An easier way to use b3d6 dice is to just set Target Numbers.

So we have a simple mechanic: Roll b3d6 to beat a target number.

The halfway point (50% chance of success) is to roll 23 or better.
For 33.33333% of success, the target number is 35
For 16.666667%, the target number is 67.

Going the other direction, around 66% chance of success is a target number of 17 and for around 83.333333% chance of success, the Target Number is 13.

13, 17, 23, 35, 67

If I wanted to imitate a d6, though, I'd use a d6. Let's use something more memorable like 15, 20, 25, 35, 40, and 65. The first five represent difficulties for mere mortals while the 65 target number is for heroic/epic action.

For those that wonder, the odds of success for rolling TN or better is:

15 - 77.78%
20 - 57.02%
25 - 45.98%
30 - 36.03%
35 - 33.33%
40 - 30.63%

65 - 16.66%

For modifier to a traditional d20 game, I'd add the INT stat to the roll, not the modifier. This will not help for epic rolls at all unless you have a stat above 27.

Okay, the chart is coming soon. 🙂