How do I calculate the speed of the interface link in OpManager's business view? 

The interface link's speed is the average of InSpeed and OutSpeed of the interface. 

i.e, Speed = (InSpeed + OutSpeed)/2

 For Example, 

 If InSpeed = 200 Mbps; OutSpeed = 300 Mbps; 

 Then, Speed = (InSpeed + OutSpeed)/2 = (200 + 300)/2 = 500/2 = 250 Mbps.