Divide each price x_t by x_0 and multiply by 100, so at t=0 the price equals 100 and at each t>0 x_t-100 is the percent change relative to t=0. Since your prof

Divide each price x_t by x_0 and multiply by 100, so at t=0 the price equals 100 and at each t>0 x_t-100 is the percent change relative to t=0. Since your profit/loss is proportional to the stock's relative change since you bought it, this is what you want to know.