Um...12 days? I'm not sure what you are going by, but I think you are going to run into problems if you are planning on 12 days.So, if the cells take twelve days to mature, and should be transferred before any emerge and emergence can vary between cells and colonies, I should start cells on the 28th of this month.
The queen takes roughly 15-16 days to develop. The egg hatches at 3.5 to 4 days. This is the age you graft at. Then 10 days after grafting (Day 14) you transfer to mating nucs, and the queen emerges a day later. (Sometimes 2 days later.)
For example, I set up a queenless cell starter yesterday, and gave them my grafting frame so they could polish cell cups. I grafted today, 4/16. 10 days from now is Sunday, 4/26. This means that I need to set up my mating nucs on Saturday 4/25.
I set up my mating nucs with 2 deep frames of bees and brood in a 5 frame styrofoam nuc. For my area, that is sufficient. Our chances of a frost are very low now. (But my Grandpa did have a few inches of snow on June 7 one year, killing all his corn that was up, and he had to replant.)
The rule of thumb is that you need drones at the purple eye stage before you graft. I saw several hives with capped drone cells the other day. I didn't open any cells to see if the drones were purple eyed yet, but I have seen purple eyed drones in years past between April 7 and April 15.
To be honest, raising queens this early is a gamble. They say it needs to be 70 degrees before queens go out on mating flights, but I have had queens get mated during stretches of weather when the highs were only in the mid-60's. Our May weather has in years past been adequate for getting queens mated, but older beekeepers tell me that May is unpredictable for queen rearing, but June is a month you can get dependable matings.
Here is a picture of a queen rearing chart. The center portion spins. You adjust the center wheel to the calendar date on the outside circle. This automatically calculates when the queen will emerge, go on her mating flights, and when she will start laying. I took this picture at the Prokopovych beekeeping museum in Ukraine.