Variable DR, Damage Reduction, & Damage Penetration

Here are three attempts to apply the basic trait of "Damage is reduced to 1/2" within game mechanics. I'm not particularly fond of any of them, but if I had to choose I think I'd go with the Damage Reduction advantage, followed closely by Variable DR.

Damage Reduction
30 points/level
    Damage you take is reduced, possibly to zero. Divide penetrating damage by 1 + your level of Damage Reduction (drop all fractional damage).

    If you have this advantage at level 10 damage is reduced to zero instead of dividing it, however level 10 Damage Reduction must be combined with a Limited Defense limitation (p. B46).

Special Limitations:
    Limited: Your Damage Reduction applies only to certain forms or damage types. See Limited Defenses (box, p. B46) for details.

Variable DR, a variation on Damage Resistance
You have DR equal to your level of Variable DR times the number of dice of an attack. This costs x10 the normal cost of DR (50 points per level), and you can only purchase up to DR 6 (which gives you DR 6 per die, effective immunity to damage). If you do take DR 6 with this variation you must take either the a Limited Defense or the Penetrating Damage limitation (below).
Example: You have Variable DR 3. Against an attack doing 9d damage you have 3x9=27 DR. Against an attack doing 2d damage this provides only 2x3=6 DR.

Special Limitations:
Limited: Your Variable Damage Reduction applies only to certain forms or damage types. See Limited Defenses (box, p. B46) for details.
Damage Penetration: -20%/level, up to 4 levels. For each level 1 point/die of damage automatically penetrates your DR. Note that penetrating damage cannot be greater than damage rolled. This limitation can be applied to normal DR too.

Special thanks to Eljay451 who first came up with the basic idea for this over on the Steve Jackson Games Forums.

Damage Penetration, a new Limitation for Damage Resistance
    Your DR doesn't stop all incoming damage from an attack, a certain percentage penetrates no matter how high your DR. Subtract DR from basic damage normally to figure penetrating damage (pp. B378-379). However, also multiply basic damage by your Penetration multiplier. If this sum is equal to or greater than the calculated penetrating damage increase the penetrating damage to that amount.

Damage Penetration: x0.1, -5%
Damage Penetration: x0.2, -10%
Damage Penetration: x0.3, -20%
Damage Penetration: x0.4, -30%
Damage Penetration: x0.5, -40%
Damage Penetration: x0.6, -50%
Damage Penetration: x0.7, -60%
Damage Penetration: x0.8, -70%
Damage Penetration: x0.9, -80%

    Example: You have DR 4 with Damage Penetration: x0.5. 4 points of damage would normally be stopped completely, but a minimum of 4 * 0.5 = 2 of the base damage automatically penetrates, causing 2 point of penetrating damage. If you take 10 points of basic damage, normally 10 - 4 = 6 points penetrates. Since this is greater than 10 * 0.5 = 5 you take the normal 6 points of penetrating damage.

    In all cases round damage up. For example, if you have Damage Penetration: x0.1 and take 3 points the calculated penetration is 3 * 0.1 = 0.3, which rounds up to 1 point of damage.

Limited Defense
You can combine Damage Penetration with Limited Defenses (p. B46), allowing DR to work normally against most types of damage but certain types tend to penetrate. Apply the appropriate Limited Defense limitation on the selected level of Damage Penetration to figure the final limitation value (see Limited Enhancements, page 111).

Example: You have DR 10 which protects fully against all types of damage except Fire/Heat, a minimum of half of which penetrates. Damage Penetration: x0.5 is normally -40%, applying the Limited: Fire/Heat (a Common type) reduces the value of Damage Penetration by -40%, giving a final limitation value of -40% * 0.6 = -24%.