As there any way to predicate the discount rate of a shipping rate prior to selecting it?

We have the following scenario

1. User adds items to the basket
2. User as a discount based upon code to the bakse that is a shipping discount (This could be a percentage out the box discount or one we've created that discounts to free if spending over a certains amount)
3. The user is presented with the shipping methods availible with their name and price.

We are currently calling the below

        /// Inheritdoc
        public IEnumerable GetPromotionsForCart(ICart cart)
        {
            IEnumerable rewards = _promotionEngine.Run(
                cart,
                new PromotionEngineSettings()
                {
                    ApplyReward = false,
                    RequestedStatuses = RequestFulfillmentStatus.All
                });

            return rewards;
        }

And then checking the SavedAmount for each of our ShippingMethods contained within these rewards to work out the discount cost or if it's free.

However this evaluation only applies if the shipping method has been selected before calling the code and as this is the point at which we are presented those options what we want to be able to for the collection of Rewards which has been returned as attached to the basket evaluate which this discout will be when it is selected.

Is there anyway of getting what the discount will be without the option being prior selected in the Cart?

Thanks,

Scott