All in all, I believe that alignment is meant to help guide role-play but that too often it dictates role-play instead. And don't even get me started on magic that influences alignment =_=.
Anyways, I made a gradient alignment chart with a grid on it to help track things, use it as you will.

Proposed System:
1.) Each character/NPC gets 100 alignment points.
2.) Assign points in terms of relative % of their motivations and influence.
3.) Minimum of 5 points in each category as no one is pure anything, people are more complicated than that.
4.) The alignment points refer to, proportionately, how a person leans in terms of alignment
Examples:
>Paladin:
>>Good: 30
>>Lawful: 60
>>Evil: 5
>>Chaos: 5
>Barbarian:
>>Good: 40
>>Lawful: 5
>>Evil: 15
>>Chaos: 40
and so on...
The purpose of the numbers is to help people who aren't very good at role-play to role-play their character by having them roll 1d100 when they can't decide what their character would do. The rolled result compared to their alignment percentages would suggest what their character might do in a situation.
Alternatively, the point of not being able to have 100% in any alignment nor 0% is that someone is always capable of acting against the majority of their beliefs or ideals, and that no one is beyond doing something in any of the categories. This is intended to help inspire more complex role-play by asserting that the character is, by default, more complex than just a member of one of nine categories.
Or...
You could just have a character's alignment be a single point on the chart since there is a grid on it already. Allows for variation and relative degree without bringing in percentages.