Since I've just been right there - which is why I haven't posted for several days - seems like a good topic.
Situation: trying to finish Book Two, two assessments outstanding, and in come the final copy-edits to check for Book One. Do I sacrifice my writing time and do the copy-edits? Do I bash away at the assessments first, and hope to catch up with the rest, while keeping a frantic head around the looming deadline?
Rule One: whatever you do, don't sacrifice your writing time.
I've broken that rule too many times and regretted it. Remember your writing time requires you to be creative, and if you're canny you've organised your life so that writing time happens when you are fully compos mentis and firing on all cylinders. Don't waste it on sweating the small stuff.
Assessments (which happen to be my other main activity) require an editing head, not a creative one. Likewise reading over and editing your own work, including copy-edits. You could label research and plotting under the same heading. You can think in your sleep!
The trick is to sandwich all the other stuff in between life commitments. Hold your creative time sacrosanct. If you don't, the book won't get written and you'll get more frantic. Plus the next time you get down to it, you'll probably find you have to read over half the book before you can start because it will have leaked out of your consciousness and will need reviving.
Hold on to, and use your writing time, and you'll get to the end.
Memo to self: Could you please for once take your own advice? Do you realise your deadline is almost in your face?!!