"about 10%" becomes about 15% to buy. That's from spot price up. When you want to sell it, it will become "about 10%" from the spot price down aka about 15% in

"about 10%" becomes about 15% to buy. That's from spot price up. When you want to sell it, it will become "about 10%" from the spot price down aka about 15% in reality. Now do the algebra.