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Is it a right way to calculate wan bandidth for the requirement or is there any other way to do ?
It's "right" as in not being wrong, but might be suboptimal.
Again, when told an application operates with 256 Kbps, it helps to know whether this is a constant bit rate or an variable bit rate. If the latter, does the 256 Kbps allow for bursting to keep average queuing latency down or is it just an average consumption? If the latter, you may need more available bandwidth than just the average usage to minimize queuing delays.
Besides possible queuing delays, there's also serialization delay. For an interactive application, you might find a single flow operates much better with 1 Mbps than 256 Kbps, because it takes each packet/frame less time to arrive.
Lastly, in real networks, when you talk of "branches", distance based latency might be a problem. An application that only uses 256 Kbps might perform very poorly even it it had a gig of bandwidth because of this.
So, again, what you did is fine, and is often used, but in real world networks it's often seen that delivering optimal performance to "branches" can be more complex than just X bandwidth times Y instances.
