I’m assuming you have non-dimmable lights. I also switched from T5’s to non-dimmable LEDs.
What I did was attach four fir strips to my stand with them pre-marked every couple inches. I started with the lights all the way up at the ceiling and each week I dropped the lights down to the next line. If you look at the strips at the far end of the tank, you can see the lines on the boards.
Eventually I got down to about 12 inches off the water. Worked well.