<<Do thieves recover their CONCENTRATION at a linear rate similar to Magic 3.2?

Yes.

You get 2% concentration regeneration each pulse. Because of rounding, this effectively means exactly one point of concentration regeneration for every 50 concentration rounded down. E.g. at 249 you will regen 4 concentration every pulse, but when you hit 250 concentration you will regen 5.

Regen pulses are every 6 seconds. Since khri pulse every 12 seconds this means you get two pulses for every khri pulse, making it fairly easy to calculate how sustainable various khri in combination are to maintain.

This can be sped up by sitting, although I haven't tested if it is an increase to the pulse rate (very unlikely) or to the pulse size (far more likely.)

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I know this math that you provided can make plunder even better (if it is bumping you over that 50 concentration hump). Is there a way to see exactly how much concentration you have while running plunder? When I check it with the "info" command it just changes from 280 to 280+.

Use FOCUS. It will tell you your adjusted maximum.

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