If you're adding flavour ingredients, use a one cup measure. More than that will not allow the dough to come together enough to form into a scone - it will be too dry. You can add more than one dry ingredient, just be sure the total amount is one cup; for instance, for my Toffee Almond Crunch scone, I used 1/2 cup sliced almonds and 1/2 cup toffee bits.
Lastly, if you're adding lemon, like I did for by cranberry and blueberry scones, just add freshly grated lemon zest - that's all you need and you will not need to change the amounts of any other ingredients. About a tablespoon of freshly grated lemon zest will give you lots of lemon flavour!
If garnishing with coarse sugar, whisk an egg with one teaspoon of water. Brush the tops of the scones with the egg wash and sprinkle the desired amount of sugar on top before baking.