MightStart by looking at their Strength Attribute, then compare to the chart. Then check Immortal Rank -- if it's below Empyreal, consider it a Tied Class.
FortitudeFor this one, simply find the Immortal Rank and compare with the chart.
EgoFor the attribute governing mental abilities and powers, find their actual Immortal Level (a numeric value) and consult the chart below.
ProwessFor tactical, strategic, and combat ability, consult the Immortal's natural Armor Class (which is actually based on the Immortal Level) on the following chart:
And that's all I have for now.
Observations and Possible Adjustments
Might & Fortitude are somewhat compressed in comparison to the Ego and Prowess Attributes. Perhaps I should expand their Class ranges a bit to give a more expansive feel to the power rankings of these abilities. Doubling them might work.
If left as is, however, it would seem that Fortitude and Might are compressed in terms of ranges, perhaps suggesting that the gaps in the costs should be higher. A more close distribution of Primary Ability costs are suggested by the Ego and Prowess classes. But maybe that's just me.
It may help to start converting some Immortals next, to see how it works out.