You have to try... buy at least two or three 35 cm cheap PVC bars and drill them. Ideally the total surface of all the holes should be similar to outlet pipe surface. I did the calculations, but as we don't usually have precision drillers at home, you work with something that is more or less what you need. This calculation will give you an rough idea of both the number and holes surfaces. Then you should try different variations (one drill more, one less, etc.).
Once you know what works best for your tank, you can buy an acrilyc spraybar and replicate your drilling pattern... in my case I am too lazy and I still have the clear PVC bar 😉
Jordi