Hello,
Velocity is the servant of mass flow rate. The purpose of flow and distribution is to move large masses of water not only to carry away debris, but to bring new water with high concentrations of nutrients and CO2 towards the plant, because the leaves will absorb the nutrients in the surrounding water and will deplete the volume of water within their immediate vicinity. Moving the water brings fresh supplies and reduces the thickness to the stagnant area adjacent to the leaf. You don't need high velocity but you need high mass flow rate because there is a lot of water to move, which is distributed over a wide volume.
The spacing and number of holes in your spraybar is determined by your tubing ID and your length of spraybar. There is no fixed distance or size that you must adhere to, but what you should do is to ensure the the total amount of surface are of your holes combined should be greater than or equal to the cross sectional area of the spraybar ID. So a good place to start is the calculate the area and then pick a number of holes. That means each hole must have a certain size and those holes will be distributed evenly across the length of the spraybar. Of course you make room for fittings and other sundry issues, but that gives you a start. If you calculate a spacing or number orf holes that is an absurd number then reiterate the procedure and re-do the calculations, this time picking a different number of holes or hole size.
Different things can go wrong when you test the bar. It might be that the flow rate from holes at the beginning is different that the flow rate at the end. If the holes are too small they water might be too high a velocity, of the holes too large the jets might not quite reach the front glass. So you have to play with enlarging the holes or modifying the length of the bar. Use a few pieces of PVC with different drilling and hole patterns to get it just where you want it. Also note that you don't have to chock the filter full of media, you can use half as much media, which will improve your flow rate. Some people enlargen the holes if they fell that the velocity is too strong, and this works just as well so don't worry too much. I just show as a general rule of thumb to try and get the jet streams to make it to the front glass, so you do need to experiment a bit but PVC is relatively cheap (in most countries anyway).
My largest tank is around 60 cm deep and I didn't have any difficulty at all when adhering as close to the 10X rule as I could. Make sure that you do not reduce the tubing ID too much with reducers and so forth along the water circuit.
Cheers,