You have more than one option for handling days like this:
1. Give the shot as usual and leave her with plenty of food. It works for most cats, but it can be a little more worrying for you especially if she usually has someone home with her every day.
2. Quick and easy - simply skip the shot. Most of us have to do that occasionally. No stress for you - it means you can simply go and enjoy your evening out without worrying about her numbers.
3. You can give a half dose. Problem with that is that, sometimes with the depot insulins, a half dose doesn't make much of a difference until the following cycle so it doesn't guarantee you won't get a low number.
4. Depending on schedule flexibility, you could give her dose whenever you get home and adjust her time back to her regular shot time gradually over the next few days.
Which of those is the best option for you really depends on your comfort level with shooting and leaving the house. I would agree though that getting a test around +11 so you can see at pre-shot what direction she's heading in would help. You can get both tests and still decide you're not comfortable shooting - for New Year's Eve when there may not be many people here and you want to have a night out, I don't think anyone would blame you for wanting to simply skip one dose out of the year to have an enjoyable night out. With our diabetics here, if I've worried about something like this in advance, they've had a habit of throwing me an unusually high number on the day so it turns into a complete non-issue...you might be lucky enough to have that happen and just be able to give her full dose too.