What device do you and others normally view your photos on?
I’ll be honest, on my phone and my laptop I can’t tell a difference between the 230 and 300 pictures. At full resolution on laptop the difference between the two is noticeable, but normally what I would ascribe to high ISO or a bit un-sharpening to hide pixel noise. It doesn’t scream “out of focus” to me.
If you’re not showing your images to people at 100% size on monitors, I really wouldn’t worry about it too much. On the average phone, tablet, or laptop screen the softness won’t be noticed by many. For portrait work, most people will appreciate it.
If you really want to see where the sharpness falls off, you’ll need to spend some time testing. It’s similar to the ISO testing I mentioned here: Photography Gear: The Camera & Lenses Thread - Hangar Flying - Mudspike Forums.
Find a good well-defined target with plenty of contrast for the autofocus (or something you can easily focus if you’re pulling focus manually), and plenty of detail to show a fall off in sharpness. Setup on a tripod, use a remote shutter release, and shoot with mirror lockup to minimize any camera shake.
You can do the quick version or the long version.
Quick version - Pick a medium aperture setting like f/11 or f/16 (usually these will produce the cleanest images, but not always). Start taking pictures from one end of the focal range to the other in about 25mm increments. Once you find out where you see softness, go back and go it in 5 or 10mm (whatever is the smallest adjustment you can repeatedly make). Then just like in the ISO test I linked above, show someone else your pictures on whatever you expect people to be viewing your work on, and don’t tell them what to look for or when. Ask them to find when “the difference starts.”
Long version - Start out at your widest aperture and shortest focal length. Make a grid for apertures and focal lengths, I recommend 1/2 stops if your camera will let you, or whole stops if not, and 25mm increments on focal length. Shot the grid. The aim here is to find your best settings, as the aperture at either end of the spectrum usually is soft and can have aberrations. You’ll probably find an area on the grid that looks pretty good (say f/11-f18 and 175-225mm). Take those boundaries and setup another grid in 1/3rd stops and 5-10mm focal length shifts, shot that grid. You’ll find the true sweet spot of a lens this way. If you really want to be thorough, now do the ISO test with your best aperture and focal length settings.
Both versions are fairly quick to shoot. Going through all the pictures can take quite a while, but it’s something you can do from the comfort of your couch or office chair.