I'm sure there is a formula somewhere, but if you are going to shoot totally in manual then its liable to be trial and error. Everything is dependent on conditions, your light, the subject your shooting (stationary vs movement), the depth of field your require, etc. Just remember your photographic triangle of shutter speed, aperture and ISO. If you adjust one, you may have to adjust another. Prior to your camera breaking, if you were shooting in the semi-automatic mode now you will have to make the adjustment the camera was making for you. If you look at the EXIF from your past photos that may help to give you a starting point.