Maitree Express: মৈত্রী এক্সপ্রেস - দুই বাংলার সংস্কৃতি ও ভাতৃত্বের মেলবন্ধন - Joydeep Roy

There is no predefined algorithm.
When a new train is announced, the rake maintenance and bulk profit share is assigned to a railway zone which is called as home zone.
Now RB give power to this home zone to decide till which station they wish to provide GNWL quota and by how much seats (Number of seats depends upon total number coaches for that class).
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Eg: A new train 18243 BSP-BGKT is going to start. Its home zone is SECR BSP div. Now its the sole decision of SECR BSP high authorities to dedicated how many seats for BSP stations and from which station the other quotas RLWL-PQWL will start. But yes sometimes special expectations also occur like some other zone from which the train pass by puts a special demand infront of home zone. Like consider 18243 has GNWL for BSP , till Gondia. Now Nagpur is in RLWL. CR NGP div puts up a demand that they wish to have the GNWL as that of BSP (Bcoz in NGP there are huge amount of Rajasthan people) and RB also agrees, then BSP div can shift over the GNWL to NGP also.
So there is pre-defined algorithm but it all depends upon Zone discussions , people demands, number of total coaches and political pressures.
PS: The above given example for 18243 is just for reference.