How do you work out the Zenith Angel of a star?

When given the declination of a star as it crosses the meridian for an observer at a given latitude, how do you calculate the stars zenith angel.How do you work out the Zenith Angel of a star?Zenith angle is that angle between the star and zenith. The celestial pole sits at the angle of your latitude. So if you are 35S, then your South celestial pole is 35 above your S horizon. If the star is at decnation 35S, then it's Zenith angle is 0 when it crosses the meridian