The criteria adopted by Fiqh Council of North America (FCNA) and Islamic Society of North America (ISNA) are that the month begins, if (1) Moon is born before Makkah sunset, and (2) moon sets in Makkah after sunset.
Proposed Makkah Hijri Calendar can be used as a global Islamic Calendar. It is based on the criteria that crescent is calculated to be visibile by naked eye in Makkah or anywhere in the world west of Makkah before the Fajr time in Makkah. This can be achieved by converting time interval into the degree interval defined as the Limiting Horizon - LH), then the new month is considered as begun, referred to Makkah. Obviously, if there is sighting before the Fajr prayer in Makkah, at some Intermediary Horizon -IH , then the month will also be considered as begun.
Moonsighting.com Suggested Global Islamic Calendar (Based on "Ashshamsu wal-Qamaru Bi-Husbaan: Qur'an 55:5")
Since a calendar has to be such that it must be based on certain rules of calculations. Therefore, Islamic calendar should also be based on some calculations, and not having to wait for sighting. Moon-birth is the only thing which can be calculated with much accuracy, so it can be used. Islamic calendar should also follow the day of the week convention so that on a Friday, the whole world would have the same Islamic date. Remember, International Dateline (IDL) has been universally accepted even by Muslims as the beginning of every day of the week. All Muslims pray Friday prayers on the same day that starts at IDL. Islamic Calendar should also follow the same convention of the day, so that day/date is the same all over the world. Such a calendar could be used globally, so that the whole world can start Ramadan or Eid on the same day.
Based on the beginning of the day convention at International Dateline (IDL), if the moon is born before a day begins at IDL (just East of IDL at 0:00 of a day), then the month should begin on that evening everywhere in the world, otherwise the month begins on that day after. This rule is not only simple, but has the consideration of Imkaan-e-Ru'yah (visibility possibility), somewhere on the globe. The moon born before that day begins at IDL would be about 18 hours old somewhere on the globe and Imkaan-e-Ru'yah exists at that age. Therefore the month can begin on the evening of that day, such that the 1st day of the month would be the next day.
This prinicple can be stated as, "If the moon is born before 12:00 UT (0:00 @ just East of IDL at 179°59' West longitude) then the month begins at sunset of that day everywhere in the world, otherwise the month begins on the next day's sunset everywhere. This suggested Global Islamic Calendar is so easy to make that no calculations are required. All that is needed is New Moon Birth (Conjunction) time which is easily available from many sources on the Internet.