Normally, the Magery advantage stops at level 3. In a high-powered campaign, however, it may be desirable to allow Magery 4+. The following is an optional rule for such "extended Magery":

First, the ability to cast spells in low and normal mana areas, sense permanent magical items on an IQ roll, read scrolls, use "mage-only" items and sense changes in mana level becomes a separate 5-point advantage called "Magical Sensitivity" or "Magery 0." This is obvious to anyone using an Aura spell, and prohibits you from having Magic Resistance. It gives no IQ bonus to learn spells and does not allow you to learn spells that require Magery 1+.

Magical Sensitivity then becomes the prerequisite for actual levels of Magery, which are now treated like racial attribute bonuses; i.e., they use the usual attribute cost progression, but with a starting value of 0 instead of 10. So Magery 1, 2 and 3 are treated as +1, +2 and +3 attribute bonuses and cost 10, 20 and 30 points respectively. Adding the 5 points required for Magical Sensitivity brings the total cost to 15, 25 and 35 points -- just as in GURPS Basic Set.

This leads to the following cost progression:

One's Magery bonus adds to spell levels, the IQ roll to detect magic, Thaumatology skill, et cetera as usual. Magery also fulfils prerequisites, and mages may develop spells that require Magery 4+ as a prerequisite.

Spells limits can be exceeded: Each level of Magery above Magery 3 allows a mage to exceed the usual fatigue limit of a spell by one increment. For instance, a mage with Magery 7 (4 "extra" levels) can spend 18 fatigue to cast a PD 9 Shield spell, spend 7 fatigue to create a 7d Fireball, or spend 8 fatigue to heal 16 points of injury with Major Healing.