How to set calendar when unavailable only for certain hours?

How do you all set your calendar when you are available for most of a particular day, but not for a certain set of hours? I usually mark myself available so potential clients will contact me, but if the timing they need is in the few hours that I am unable to be there, when I turn them down and archive the request as "I wasn't available," my scores go down because my calendar wasn't "up to date". Is there a way to avoid this? I'd rather not lie when choosing a reason for archiving requests. Is it better to set myself as unavailable for any day I am not 100% free and just hope clients still contact me?