I hope everyone remembers not to buy a $30 shirt "on sale" for $29.95! Awww come on! Most people could almost do that in their head, 30 / 1.2 = $25 :) 1 T-Shirt @ $30, Portfolio control = $30, #Shirts = 1 Next buy price PC / ( #S x ( 1 + SAFE + Minimum Trade Size ) ) = 30 / ( 1 x 1.2 ) (assuming 10% safe, 10% min trade size) = £25 next buy price Next Sell Price PC / ( #S x ( 1 - SAFE - Min Trade size ) ) = 30 / ( 1 x 0.8 ) = $37.50 Changing Min trade size to 5% next buy price = 30 / ( 1 x 1.15 ) = $26.09 Most people wont however own just 1 shirt (share) 100 shirts, $30 each, next buy price : Portfolio Control = 3000 #S = 100 3000 / ( 100 x 1.2 ) = 25 (assuming 10% safe and 10% min trade size) 3000 / ( 100 x 1.15 ) for 10% safe, 5% min trade size = 26.09 Doh!