Weight won’t budge at 12:12.
7pm to 1pm is 18:6…7 pm to 11am is 16:8…7pm to 3 pm is 20:4…You’ll need to do one of those. I don’t think this works if you eat breakfast and dinner. One meal a day is best.
So much bad advice here. Dude, just do a 12 hour fast in the middle of the day. It’s fine! Read up on time restricted eating. Count your calories. I’m guessing a shake plus dinner is gonna be 1600 cal if you have a 1000 cal dinner which is pretty darn big. Succeed.
You’ll be consuming fewer calories by skipping lunch. I know ketosis starts at 12 hrs on average, but after a small meal, it should happen a lot quicker. I think your plan will work. I do something similar with lattes when I feel like treating myself.