In this case, less is more. There's no need to remove too much material on the first cut. It's easier to try a smaller amount and test it out. Then, if needed, cut some more. Generally, you should be able to see over the upper edge of the shield when seated comfortably. Since there are no hard-and-fast rules, you can try one of the following methods as a guide to determine just how high you want the shield to be.