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I've cracked the code.
Gen 1 remakes in Gen 3. (1.5 is half of 3, and Pokemon always rounds down)
Gen 2 remakes in Gen 4 (2 is half of 4)
Gen 3 remakes in Gen 6 (3 is half of 6)
Its always double its original generation. Expect Gen 4 remakes in Gen 8, Gen 5 in Gen 10, etc.