A UVI meter (Solarmeter 6.5x or DIY equiv.) is the only way to tell for sure.
Some "guesstimate" 8-10" from UV bulb to basking site (see bulb mfrs. sites for recommendations), but my UVB runs a little hot, so it's 12-13" to get a UVI of 3.
If that new fixture includes a new UVB bulb, remember that these T5 bulbs typically require a burn-in period of 50-100 hours when they will put out higher levels of UVB. Some let them burn for a week (12 hrs/day) or temporarily lower the basking site by a few inches. Again, a UVI meter is really the only way to know for sure. They're a few dollars, but will pay for themselves in time just in not replacing bulbs too soon, but also in peace of mind that everything's as it should be without guesswork.
