Well:

Powering-up items ALL boosts power by 20% (except Wise Glasses and Muscle Band which boosts by 10% and Life Orb by 30%)

Base power of Earthquake is 100, Torterra's Earthquake is 150 (STAB), holding Soft Sand or Earth Plate 180, holding Life Orb 195, Muscle Band 165.

0% of all are 0 power

25% of 100 is 25

150 is 37,5 (38?)

165 is 41,25 (42?)

180 is 45

50% of 100 is 50

150 is 75

165 is 82,5 (83?)

180 is 90

200% of 100 is 200

150 is 300

165 is 330

180 is 360

400% of 100 is 400

150 is 600

165 is 660

180 is 720

BTW, most powerful attack without counting stats is Blast Burn:

150 Blast Burn

*1,5 STAB

*1,5 Sunny Day

*1,5 Blaze

*1,3 Life Orb

*2 Critical Hit

*4 on Parasect

*1,25 by Parasect's Dry Skin

It's base power is 6581,25 (6582?)