Depyrogenation tunnel belt speed calculation

Why cos 30 degree is used in tunnel belt speed calculation & whats its value and also what is temp.inter prob variation accepted during thermal validation of tunnel

Because at cos 30 (cos 30 is only for width) only we can see degree of deviation. If sin 60 we can say it is for length.
If we arrange vials in length & width wise,only width will release the gaps,so we are taking cos 30.