how to figure disending grade?

Question by deadmans hand: how to figure disending grade?
I am building a grade on a model railroad and am trying to stay between 1.5 & 2.0 percent on the grade . It will fall a total of 4.5 inches in roughly 25 feet . I dont think it can be done in that short a distance with out it being over 6 + percent ,any help appreciated .

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