By the rule of thumb -

Rule of thumb (phrase):
English Meaning: A broadly accurate guide or principle, based on practice rather than theory.
Bengali meaning: অভিজ্ঞতা ও অনুশীলনের পর প্রতিষ্ঠিত পদ্ধতি বা প্রক্রিয়া।

Example - A good rule of thumb is that a portion of rice is two handfuls.

Source: Oxford Dictionary & Bangla Academy Dictionary.