Is there a way to display how much of the hard drive's read/write capacity is being used? I am interested in understanding how much of my 7200 rpm hard drive's capacity is being used, because sometimes performance is very slow and choppy.

The Disk tab in Activity Monitor gives information about Data read/sec and Data written/sec – but is there a way to display such information relative to what it's capable of, e.g. a percentage measure.

Maybe there is a separate utility that could assist with this? Or perhaps, for technical reasons, this is not the way you measure drive performance?
You're reading exactly what you want. A percentage throughput would not give you more information, just less accurate information as to the state of your I/O. Your drive should be pushing about 150MB/s at the most (Spinny disks aren't too fast in general, and worse when mostly used).

I am interested in helping with your main issue of slow and choppy HDD performance. If this is a MacBook or MacBook Pro unibody non-retina, you may have a degraded SATA cable. My first one went out after 2.5yrs in a 2012 MBP. A friends' went bad after five years in her 2010 MBP. It's very common across the support forums too. If you feel data access has gotten slower, or you've started having issues with freezing, spinning beach balls, and/or crashes and reboots, swap the cable before the drive, or both together. Not changing the cable led to my frustration over the slowness of two day shipping twice too often now.