Different formation have different Porosity and Water Saturation cut-offs. They are usually decided upon after getting some production data from nearest field. Sometimes very low porosity can still results in production, particularly when producing a gas field.
Usually however, in case of a sandstone reservoir the Shale Volume cut-off is 35%, the Water Saturation cutoff is 50-60% and the Porosity cut-off of 3-5% are used. This can act as a base for initial calculations, but needs to be re-evaluated on a case to case basis.
Cut-offs are very important for calculating the Net-Pay and usually follow from interpreting electric logs (resisitivity usually). No software is available and it is mainly based on the interpreters vision and experience. When starting of it is some times good to change the parameters and see the effect is on the N:G ratio.