Well for starters, this is a desert species not a tropical one, all living things UVB bulbs are not sufficient enough, T8 UVB bulbs can not be used unless a specific kind and mounted inside underneath screen at about 6-8 inches from the basking spot.
Just to point out plastic and glass (solid objects) block out all UVB, sadly some fixtures come with a plastic guard over the bulb which means the beardie will get no UVB with the plastic over the bulb.
Screen also blocks about 40-50% of all UVB.
Only brands of tube lights that can be used perfectly fine are Reptisun and Arcadia. And even then you can't just get any bulb as some are better than others, some produce a higher percentage, some can't be placed on screen, and some are just worthless.
The minimum for on top of a screen enclosure is a Reptisun T5 HO 10.0 UVB. (10-12 inches from the basking spot)
The minimum for inside the enclosure is a Reptisun T8 10.0
UVB bulb. (6-8 inches from the basking spot)
I suggest going for a 22" or a 34" Reptisun T5 HO 10.0 UVB tube light with a reflective fixture.
Other acceptible UVB tube lights,
Arcadia D3+ T5 HO 12% (10-12 inches from the basking spot)
Arcadia D3+ T5 HO 14% (about 16 inches from the basking spot)
(When I say 10-12 inches away from the basking spot that Is basically how far it can be from it and still produce a decent amount of UVB)
Just in case, the minimum for a baby bearded dragon is a 20 gallon, that would last up till about 6 months of age were you would then need a 40-50 gallon which will last you a bit until you need the minimum of a 75 gallon enclosure when they turn about 12 months old.
You could probably use the 22" for the 20 gallon and 40-50 gallon enclosure but for a 75 gallon I recommended going for a 34" or a 46"
UVB bulb.