The crucial words here are "excess steam".
Only you can decide what that is. Making it the maximum your boiler can produce would seem to be not economic. but that's up to you.
What's the downside of undersizing it?
I assume there is a separate relief valve venting straight to atmosphere?
Yu can see the entirety of your system, we can only see the minimal amount of information you've given us, so you will only get back a minimal amount of comment....
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.