Them being on Day 20 made a simple question hard. Normally I’d say now so she could go through the hatching process. The hen and chicks talk to each other even before the chicks hatch so she comforts them and they prepare her for the change. Plus the broody does all the work.
But at Day 20, you might have some pipping already. If you have some pipping, it’s possible you could shrink- wrap them moving them under the broody. It’s possible, not an absolute guarantee you would have problems, but I find it better to leave the eggs alone at that stage.
I think you did right by waiting a couple of days to make sure she is truly broody. I’ve had some that acted broody but just didn’t stick with it.
What I think I’d do in your circumstances is to wait until your hatch is over, then put them under the broody at night, shortly after dark using as little light and commotion as possible. It needs to be dark enough she can’t see what is going on while they are under her. The chicks need to be as young as possible so they will bond with her and she is more likely to accept them. She should feel and hear them overnight and accept them the next morning. This doesn’t always work but it does the vast majority of the time. Still, you need to check about first light the next morning to see how it is going. You may need to use that brooder if she doesn’t accept them.
Broody hens cannot count number of eggs, number of chicks, or number of days. Plenty of hens accept chicks like this after being broody only 2 or 3 days, but the rejection rate is a little higher if they have only been broody a few days. I think that some broodies haven’t really kicked into full broody mode so that may affect this. Them being broody is caused by hormones and some chickens may be a bit slower than others for those hormones to really kick in. Still, I think you have an excellent chance at success.
Good luck! You’ll love seeing the broody taking care of her chicks.
