Here's me putting your values into your formula:
>>> # Yesterday I invested 3.35 BTC.
>>> V1 = 3.35
>>> # Right now I have 3.32925858 BTC
>>> V2 = 3.32925858
>>> Percentage_change = (V2 - V1) / V1 * 100
>>> print Percentage_change
-0.619146865672
Now you try?