Now, for the equation. First, let's check what exactly is going on with our avatar.
It's moving for the whole length of the circle, our circumference, and, while doing so, it completes a full rotation on its own axis.
As we already know, the length of the circle is calculated by multiplying the diameter -- or twice the radius -- times pi (2 x π x R) and a full rotation consists of a 360º turn.
Also, we know that both events happen at exactly the same time or, in our case, a given number of ticks (Δticks). Which can, thus, be calculate by dividing the total distance (2 x π x R) by the distance run in a tick (spd) and by dividing the total rotation in degrees (360º) by the amount of degrees turn in a tick (rot).
Using both equations, we can create a third one that shows us the relation between an avatar's spd and its rot, allowing us to calculate any of them if we have the other.
This equation, "translated" into Construct 2 is what we used on our little tutorial:
Hope it helps!