What point system is most accurate for relative training stress?

I have been trying to write a training schedule for a fall marathon with marathon/tempo segments in the long/longish runs so that they produce stress that is roughly equal to a comfortable 20 miler.

I have been using the following:

Points per mile:

5-6 E

7 T

8 I

9 R

On this scale a comfortable 20 miler is worth 120 points as is an 18 miler with 10 @ MP. Does this relative scale seem to be accurate? Interested in how others approach this.