TL,DR: Constellation Spell Penetration is ½ what it states and Sharpened Trait is ×0.02 what it states. So long as my calculations are close to correct.

I have reverse calculated the Armor/Spell Resistance Mitigation formulas and calculated the Spell Resistance of V10 Giants in the Rift to be approximately 23250. This value has been constant through various other Damage calculations I have performed.

The formula I have used is:

100 - ( ARMOR - ARMOR ÷ 50 ) ÷ 630 ) ÷ 100 = Mitigation Multiplier
*NOTE: Truncation and/or rounding is used, and my values are off by small margins.

Example:
Dark Flare does 3239 damage on Tool Tip, versus V10 Giant will only do ~2066

With this known and verified, I went to test spell penetration. I inserted this value several different places in the formula above and came up with the following as my most accurate estimate:

100 - ( ARMOR × ( 100 - PEN ) - (ARMOR ÷ 50 ) ÷ 630 ) ÷ 100

This formula provides reasonably accurate (to within 1% difference) from in-game values for damage. When applying the math with reported values however, it appears that several stated % values in-game are not what they seem. Specifically, Spell Erosion in the Apprentice and the Sharpened Trait on weapons are both off, the former provides only half what it states, and the latter provides only ×0.02 (yes, really) what it states.