my recipe calls for two finely grated lemons, I prefer using lemon juice, how much juice should I use?

Also, lemon juice is high in acid, which can wreak havoc on the other ingredients, producing less than satisfactory results - including ruining the dish, if the lemon juice causes curdling or interacts in a funny way with leavening agents. ;o)

You won't get the fragrance and flavor from juice that you will from zest. Lemon peel (the yellow part, not the white pith beneath) contains all of the essential oils that really shine through in baked goods and other foods where the acidity from the juice is not desired.

Lemon juice will not give the same flavor and depending on the recipe could cause curdling. Either buy an organic lemon and use the grated peel or use lemon extract. Your best flavor is with the zest.