Velocity of a Pendulum

Homework Statement

Have to find the velocity of a pendulum at various points around it's course.
r is 4.2 m
g = 9.81
Angle changes

Homework Equations

Ok, so i went looking for an equation and found this.
v = root 2gr(1-cosX)

The Attempt at a Solution

I pretty much plotted the above equation in y = on a TI calc, whereas X was the angle. I have results - Except i'm worried about the frame of reference. The website i found the equation gave a frame of reference. But i'm concerned that this is wrong. Could someone point me in the right direction please? Thanks

The results where as follows;

45 degrees = 4.9
90 = 5.1
135 = 11.9
180 = 12.8

These results work if the angle is taken fat the top of the semicircle - Where the velocity increases as it falls towards the earth. But the FOR that was given, it suggested that the angle be taken as the bob is motionless down the bottom and taken left to right. Help?

$$v = \sqrt{\frac{2gl}{m} (\cos(\theta) - \cos(\theta_o))}$$
here, 'l' is the length of the string and $\theta_o$ is the angle made when the pendulum is at it's highest.