Substrate = base fertiliser + gravel
For me, I would use about 1" (at most 1.5&quotfor the top gravel. This would enable the plant roots to get to the base fertiliser. 10cm substrate is OK. Some people lay gravel, then base fert and topped up by gravel again to make their substrate.
BTW - my substrate is 3". That's about 7.5cm.
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