lower end should be as low as possible. ie no heating and well vented, if the lizard is too cold it can go down the other end and vice versa. in summer time i had to use air con when had a number of enclosures operating. like 30-40C all round the lizards can not escape the heat if need be.
i just used, 40, 60, & 100watt spotlights from bunnings. ceramic is ok but not natural in the sense they don't put out light. also a bugger if they blow cos unless you check temps you may not notice for some days.
when brumating they can get down to about 10C and seem no worse for wear, suspect mine got lower for short periods of time.
generally room was down to about 10-15 at night and 20-25 during day.
15C seems to be the recommended temp for brumating, still seems pretty warm to me.
regarding basking temps. if too hot lizard will sit off to one side when basking. too cold will sit right under heat source.
ive only got two oldish beardies now. don't even check temp cos misplaced my temp gun lol. above method seems good enough for me.