Hi William, first you want to factor your original hydration percentage (60%). Next you want to calculate how much you would need to increase the liquid to bring it up to the requisite 75% hydration, which gives you the amount of extra water you need to add to your recipe. In your example that would be 150g of additional water. We're on the same page up to this point. 

I think where you may be getting confused is the next step, which involves calculating the flour/water to go into the tangzhong, and then deducting these amounts from the increased hydration recipe: 5% of your flour would be 50g, and that would make the water 250g for the tangzhong starter. These amounts of flour and water are then deducted from the increasd amount of water (750g) and the original amount of flour (1000g). This leaves you with 500g of water and 950g of flour for the dough portion of the recipe. Remember that the tangzhong starter is going to be adding further hydration, so no need to worry about the dough hydration at this point. 

I hope this helps! 

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