How far should the UVB tube be?

The honest answer depends on your exact fixture, your exact tube, and what the light passes through on the way down. Leave any of them out and the number is wrong by inches. That is a real calculation, and there is already a free tool that does it properly, so we will not give you a worse version of it.

The tool we use ourselves

The Reptile Lighting Tool

Pick your fixture, tube, screen and enclosure and it works out the UV index, watts and lux for your exact setup, for free. It is built on the same measured approach our guide teaches: UVI at the basking site, not wattage on the box.

Open the Reptile Lighting ToolFree · opens lightmyreptile.com

Before you measure: the targets

Target UVI 3 to 5 at the basking site, ideally 4.5 to 5, measured rather than assumed.
Coil bulbs do not cover a basking zone.Use a linear T5 HO tube around two thirds the enclosure’s length.
Mesh matters enormously, screen tops block anywhere from 12 to 70% of UVB depending on the brand. The tool accounts for it.
Replace tubes on the maker’s interval. Output fades long before the light looks dim. The free tracker has an 11-month reminder preset so the new tube is in hand before the old one is due out.

The full reasoning, with the measured tables and named sources, is in chapter 3 of the guide.