Just keep adjusting the height using whatever safe methods you can work out, and keep checking the thermometer and watching the chicks behaviour. You'll find the sweet spot in due course.
your setup looks a bit dodgy with that lamp balanced there like that. If it falls in, the whole thing will go up in flames, along with your house. Make sure it is really secure and can't be knocked off.
if you cannot afford new stuff, search the house light fittings for light bulbs that are 40 or 60 watt and swap them with the one in the brooder. Not LED or longlife bulbs though
Just try and get it balanced in the right place where they don't huddle under it and chirp with cold or lie down as far away as they can get while panting and wing spreading. They should have a good mix of sleep and activity and not excessive chirping. If you can't adjust the height, then get a...