How much substrate for dirted tank, 15g (55l)?

I'll be building my 1st tank next week, i bought a 60x30x30cm aquarium, i want to do a dirted tank with lots of plants to be as low maintenance as possible and i'm watching this father fish video.

https://www.youtube.com/watch?v=eyAnFz5XeuU

He tells me firmly, 1 inch of dirt and 2 of sand. 30cm is 12 inches. so 3/4th of the tank will be just substrate.

Isn't it too much? I'm not saying that he's wrong i can't say that since i have zero experience, i'm just asking if for this 'small' tank losing a quarter of the water column or even a third since the water won't be to the rim too much?

Make it a slope?