Any creature’s power level can be determined by adding its ISP, PPE, PS, PE, PP, and Spd together and taking that value and multiplying it by ten and adding that to either the creature's combined S.D.C. and Hit Points divied by 10 or 10 times their M.D.C. value (for M.D.C. creatures only), then taking the final result and multiplying it by the creature's level of experience, then rounding any fractions down.
While these levels are different from those shown in Dragon Ball Z, I was unable to create a method of determining the power level of a character that still made sense and was at least similar to the system used in Dragon Ball Z. Because of the nature of magic and psionics, this system is not as useful for determining how powerful a magic user or psychic is, which can lead some characters into underestimating their opponents.
Example 1: An average 1st level Saiyan that is not energized has an ISP of 41, PPE of 14, PS of 23, PP of 14, PE of 14, Spd of 50, SDC of 190, and Hit Points of 31 and has a power level of 1582 ( [ [41+14+23+14+14+50]x10 + [190+31]/10 ] x1 = 1582).
Example 2: an average 1st level Saiyan that is fully energized has an ISP of 41, PPE of 14, PS of 28, PP of 16, PE of 17, Spd of 60, and MDC of 120, from SDC of 190, Hit Points of 31, and an M.D.C. bonus of 10 (190+31 = 221/2 = 110.5+10 = 120.5 = 120), has a power level of 2960 ([[41+14+28+17+16+60] x10 + 120x10] x1 = 2960).
Many people that study the Art of Ki Manipulation can change their power level very quickly, and this is especially true for Saiyans, which have the Energizing technique as a natural ability. If a creature possesses a way to rapidly change their power level, then getting an accurate reading, even from a Saiyan Scouter that displays the exact power level, is difficult and often useless. For example, as a Saiyan becomes a Super Saiyan, there power level dramatically increases, which makes earlier power level readings misleading.