So our upper leaves have been developing some burn...

Things I notice here that may or may not answer your question: 1. Grapefruit has a 60 to 70 day flowering time. You're inside of.. week 8? Which puts you somewhere close to (or in) the window. I've grown 8 Ball Kush which is rated at around the same flowering time, and it was ready in the 6th week. Because of this, the buds didn't get super fat but the quality was still amazing! 2. She LOOKS like she's close to done flowering. All the nutrients are being sucked out of your lower leaves, and possibly now uppers too. This likely means she's ending her life cycle and using up available nutrients within instead of pulling them from the soil. 3. As per #2 above, it looks like you're developing some heavy nitrogen burn? If she's at the end of her life cycle, you can feed all the nutes you want and you're just going to overload her because you're adding on top of what she's using from within. It may be time to flush for a few days. Unfortunately I think you've missed your chance to do a proper flush in soil medium (1.5 to 2 weeks!). However, it just seems to make the bud a little harsher, there's no quality effects that I can notice when that happens. Look really hard at 60x magnification to see if you can find ANY amber trichomes. Even if you just find a small handful, I'd personally go ahead and harvest. If no ambers but more than 50% of your trichomes are milky, I'd personally go ahead and harvest as well (this will give you a slightly more heady, energetic buzz). If you didn't get to flush, make sure you cure the buds for at least a week before you smoke them or it'll be super harsh. Collect kief off your trim, you should get more than enough to smoke for at least a week while you get a minimal cure! There's a demonstration of how to collect kief and quickly press it into a block on my "Gen 2" diary's harvest. From there you can smoke or dry-vape it as is (it'll burn kinda similar to resin, this is what I do with it), put it in joints (also what I do with it), press it into rosin (wish I had a press), turn it into a tincture for edibles (see GreenMachine's "Bucket of Cookies" diary for how to do this!), etc. Hopefully some or all of this information is helpful.