Your shipping rule is just off slightly, it is just like I mentioned in the previous post. For the calculation portion you have it set to 'based on the subtotal of the entire cart + 75'. The trigger is setup fine, the shipping will only be charged if the value of the cart is <= to 600. You just need to update this calculation portion to be a flat rate of 75, your rule will then read like this:
if the total of the cart is <= 600 charge 75 for shipping
You do not need to make a free shipping rule, this rule will only apply shipping if the total is equal to or less than 600.