You just do whatever is going to work best in your situation. The great thing about the rules for Mama Heating Pad is that the simpler you keep it, the better off you are! I had 15 chicks under mine, and I used the large heating pad. No set rules on the size - just make a frame and either...
Thanks, @aart All I could remember was the pseudo part, probably because it's fun to watch my little spell checker dude have a coronary while I fiddle with spelling it. <sigh>
https://www.backyardchickens.com/a/yes-you-certainly-can-brood-chicks-outdoors
https://www.backyardchickens.com/t/956958/mama-heating-pad-in-the-brooder-picture-heavy-update
I really don't like heat lamps at all, so I'm glad you thought about Mama Heating Pad. I am a firm believe in...