There are many schools of thought on this bottom time calculation . Assume this You will need enough air to bring you and your partner to the surface safely and this is to include Deco obligations . Your consumption of air during normal breathing ,Your tank size all play into the final calculation . Assuming an emergency uses more gas this is also added in . It will give your "Rock Bottom " time .
Drift dive ,cave dive or wreck will all use a different method as will the depth you are diving . The safest way too do this is dive in thirds until you know how to do the calculations .
I do not use a computer ...... Depth gauge ,SPG and dive watch . Write it down before the dive on a slate or wet notes to refer back to them .
Hal