Easy Math

(Song and Silence: A Guidebook to Bards and Rogues)

Level: Bard 0,
Components: V, S,
Casting Time: 1 action
Range: Personal
Target: You
Duration: 1 round
Saving Throw: No (see below)
Spell Resistance: Yes (harmless)

This minor magical effect makes counting and calculating a breeze.
You can estimate the number of gold pieces in a pile, the distance of a gap you might have to leap, the number of foes rushing toward you, or the like—all in a flash.
To make such an estimate, make a Perform check (DC 15).
Success means your guess is within 10% of the true figure; failure means it is off by 20/6 or more.
The exact extent of such a miscalculation is up to the DM.