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Its just looking at the PTOE by seeing how many valence electrons the element has and then using # of valence electrons - number of electrons in the lewis structure. For example nitrogen has 5 valence electrons so if it had 7 electrons in the lewis structure the formal charge of nitrogen would be -2.
I'm not sure if there is any kind of short cut to calculating the formal charge but all group 17 atoms have a formal charge of 0 when they only have one shared bond & six lone pair electrons and all group 16 molecules have a formal charge of zero when they have two shared bonds & four lone pair electrons.
Instead of using the equation, I take the number of valence electrons that the atom has and subtract 1 for each lone pair electron and one for each single bond attached to that atom. Maybe this is what you are referring to.
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