Confused about heating sources for brooder

[Ponypoor]

My room is 68 degrees (in my house). I was planning to brood them in a space that is 12sf for two weeks indoors, and then move them to a larger area within the coop itself outdoors (temps 40-50 degrees outside) for the next 4-6 weeks after that to get them used to the coop, so I think I will need multiple heat source options for both scenarios. But I can't imagine 15 chicks using a 12" heating plate for more than a week.

I am also having a confusing issue with the heat lamp anyway--see above--its been on for a whole day about 2' above a plastic tote and hasn't even reached 85 degrees surface temp. Doesn't really feel hot at all except within 1' of the lamp, but putting it 1' away is way too close to the chicks and the brooder. Does anyone know what would cause this?
I've been told it would be so hot it would melt the plastic and it wouldn't work, I could just use a regular bulb, but that's not happening at all, it's as though the 250W bulb isn't directing it's heat into the brooder but just dissipating into the air.

Just wondering how it went with your chicks and heat sources.