Scrum Master's capacity?

I  always keep team's capacity in mind and try to ensure that no one , including my PO, gets overburdened and we are able to achieve a sustainable velocity.

Why are you doing this? Why don't others keep their own capacity in mind, when they decide how much work they can take on?

I have not yet come across an effective ways to gauge scrum master's capacity utilization

That's because it is unlikely to be effective to do so. The effectiveness of a Scrum Master is not a matter of their "capacity utilization". When the team succeeds the Scrum Master succeeds, when the team fails the Scrum Master fails.

@Ian,  thanks for you reply. 

While I appreciate and like the coaching approach in your replies that want me to think about the solution myself, I am actually looking for examples that others might have come across.:)

To answer your question, I do this because scrum guide tells me that scrum master is accountable for teams effectiveness, and establishing scrum as defined in the guide. The framework itself requires the scrum master to foster an environment where scrum, as defined in guide, can materialize. 

I know there can be a debate on accountability vs responsibility, but I opine that guide doesn't forbid the scrum master in doing things on the ground and taking these responsibilities as well. 

I perfectly understand that the ultimate "nirvana" for a scrum team would be to be self-organized in every aspect, but the reality is most of the teams follow a learning curve and take time to reach that point where they capable to take decision for themselves. Till that point "someone" has to coach the team on possible ways to improve efficiencies that will work best within team's constraints. And why not the scrum master to do that initially, when the team is fledgeling ?

You've mentioned that any possible way to guage  scrum master's capacity is "unlikely" to be effective? Any examples based on which this statement is based? As mentioned in the beginning, I am looking for such possible ways. 

Till that point "someone" has to coach the team on possible ways to improve efficiencies that will work best within team's constraints. And why not the scrum master to do that initially, when the team is fledgeling ?

My approach has always been to let the team make their own decisions and I coach them to learn from it.  If "someone" makes the decisions, you aren't coaching them to do it for themselves.  I encourage the team to make their own decisions, even if that team is on their very first Sprint.  I will facilitate discussions for the team to help them come to a consensus and then facilitate discussions to inspect the outcome.  But I do not in any way give them direction on how to do it. Sometimes the best success stories are when something went wrong and the team was able to adapt. The only failures happen when no learns from the experience.

You are correct that the Scrum Guide does not say a Scrum Master can't do this.  But it doesn't recommend it either.  You have one interpretation.  But have you inspected the results of doing it that way? Has it worked for the teams you are involved with in the past?  Just as a Scrum Team needs to inspect and adapt, so does the Scrum Master.

The only "metric" I have ever used for Scrum Master capacity is whether the teams they are involved with are able to consistently deliver incremental improvements for the Products they support.  True, that is not a conventional capacity measurement.  But my interpretation of the goal of a Scrum Master is to make themselves invisible to the team.  There should not be any reliance on the Scrum Master by the team that could inhibit the team's ability to deliver.  If the teams that a Scrum Master are currently engaged with are able to consistently deliver and are not dependent on the Scrum Master, then that Scrum Master has capacity to help another team.  If not, then that Scrum Master still has work to do with the teams with which they engage.