Reptisun t5 5.0 or 10.0?? ASAP!

Hey all, we are getting the reptisun t5 bulb and had some questions! First, we have a 2 month old in a 40 gallon tank with mesh top. Now I don't know the dimensions of this tank since I purchased it and the beardy from my brother.. But estimates are: the top of his basking rock/cave sits only about 4 or so inches up off the carpet, the little branch he has sits about 6-7 inches up and his suction cup vine sits about 4-5 inches from the top corner.. Give or take on all those since I haven't actually measured!
Anyway. we need to know if we mount a fixture (without a reflector) INSIDE the tank, do we go with 5.0? If we go with the Reptisun hood fixture that you can get as a set with the bulb, do we get the 10.0 so it can sit on top of the mesh? We prefer to mount it inside... but wasn't sure which option was best! Also since we have a 40 gallon, is a 24" bulb optimal? Thanks! Need answers asap to get it all ordered!

The 10.0 T5 HO is what you need. The fixture from Reptisun come with a 5.0 so you will need to order a 10.0 for it.

If it's on top you def. need 10.0. If inside the 5.0 will be effective there.

We seen a lot of posts and videos of people using a cheap fixture with the 5.0 bulb inside the tank. (attaching it somehow to the inside glass or inside the top.. idk which!) I think we want to attach it to the inside glass but I wasn't sure if that is a good idea or if just getting an on top of the tank fixture with the 10.0 would be better.. I want to do what's best for our little beardy! I guess Walmart has a fixture that holds the t5 bulbs well and is fairly slender and very cheap but with no reflector. So we considered that for inside the tank with the 5.0 bulb since it shouldn't need a reflector inside.. Sound good? Thanks!

A 5.0 does not provide enough UVB for a dragon, it is meant for tropical reptiles.