No it's not always assumed to be 40%. As long as it's in between half and 1/3rd it's fine.
<div class='spoiler_toggle'>Spoiler: click to toggle</div><div class="spoiler" style="display:none;">Freeza's revenge
Freeza
post cybernetic enhancement power up
~ 70% power: 112,000,000
--- Super Nova: 150,000,000
~ 100% power: 160,000,000
King Cold
~ full power: 90,000,000
Future Trunks
~ base: 4,200,000
~ Super Saiyan: 210,000,000
Son Goku
post Yardrat training
~ base: 4,800,000
~ Super Saiyan: 240,000,000
Vegeta
~ base: 4,000,000
Piccolo
~ full power: 3,600,000
Son Gohan
~ base: 900,000
The Future
Future Son Gohan
~ base: 2,800,000
~ Super Saiyan: 140,000,000
post left arm amputation
~ base: 3,800,000
~ Super Saiyan: 190,000,000
Future #17
~ max power: 500,000,000 (infinitely)
~ 35% power: 175,000,000
Future #18
~ max power: 500,000,000 (infinitely)
Arrival of the Androids
Son Goku
~ base: 9,600,000
~> Super Saiyan: 480,000,000
~ Super Saiyan (sick): 320,000,000
Piccolo
~ suppressed: 8,000,000
~ full power: 400,000,000
post re-merge with Kami-sama
Kamiccolo
~ weighted: 720,000,000
~ full power: 800,000,000
Son Gohan
~ base: 7,200,000
Kuririn
~ full power: 6,000,000
Yamcha
~ full power: 4,000,000
Tenshinhan
~ full power: 5,000,000
Android #19
~ max power: 240,000,000
post ki absorbtion
~ max power: 300,000,000
Vegeta
~ base: 10,000,000
~ Super Saiyan: 500,000,000
--- Big Bang Attack: 600,000,000
Android #20 (Dr. Gero)
~ max power: 280,000,000
post ki absorbtion
~ max power: 340,000,000
Future Trunks
~ base: 9,000,000
~ Super Saiyan: 450,000,000
Android #17
~ max power: 800,000,000 (infinitely)
Android #18
~ max power: 700,000,000 (infinitely)</div>