This is contract arithmetic, not tax law
There is no statutory notice period for private employment in India. There is no statutory rule about whether a buyout is computed on basic or on gross, and no statutory per-day divisor. All of it comes from your employment contract, which is why this page asks you for the days and the rate rather than telling you what they are.
The convention we apply, and state, is a thirty-day month for the per-day rate. If your contract says otherwise the arithmetic is the same with a different divisor. Read the clause - the difference between a basic-based and a gross-based recovery is often two to one, and it is the single largest variable in the number.