Oldtimers will say - 8-12 inches distance from the crown of the plant to the T12 tube for standards, 6 for minis. But we have new more efficient fluorescents now and moving towards LEDs. So while I agree with Jeff - some plants will need more space, some - less - you still need to provide enough and not too much light at the crown level.

I am a believer in the proper tools. I cannot eyeball the light level. So - upload an APP 'Lightmeter" or similar app on your cell phone - or if you do not have an android or iPhone - ask a friend or relative do it for you. I find 250-400fc on a crown level to be a good range. Dark leaf violets will like higher number - under the tubes - and light color violets will be happy on the edges.

Since you purchased the LEDs from Amazon - and they are different from mine, Jeff's and everybody else - you need to take one measurement before you mount them. Hang the fixture 20 inches above the horizontal surface - and find the sweet spot - 400 fc under the center - lets say it will be 10 inches. You will add the height of the plant and the reservoir, space that the light fixture will take - and it is your distance between the shelves.

The light level diminishes away from the center - so if you put your plants higher in a back of the shelf for better presentation - it will work too. And you can always mobe your plants around to accommodate higher and lower light requirements,

If you are not comfortable to do the measurements right now - you can start with 20 inches between the shelves and move them farther apart if the leaves start bleaching and hugging the pots. The low light is not damaging - so start from more - and gradually reduce the height until you see that it is too much. More light - the plants will bloom more but the bloomstalks and petioles will be short. When you are close to a sweet spot - you can play with the time.

And of course one needs to take into account light degradation over time. Violet calculus. lol.

Luckily they do OK in a range of parameters. Otherwise we would be in trouble.

Several people at Buddies suggested 17 inches, and this seems more practical than 20 inches because few of the standard wire shelving sold will give four 20 inch spaces because the most popular standard height size is 72 inches.

However, in keeping with Jeff's results based math approach, all four spaces do not have to be 17 inches, some can be more, some can be less, and that way one can still have needs based applications.

You are correct - it makes sense to have closer located shelves and fartehr located shelves - so you can grow taller plants on one shelf and miniature and starters on another. Since you are an Orchid grower - sticking phals and paphs on the shelves with AVs makes sense as well - they seem to come in bloom faster. That is my very limited Orchid experience.