guess I never left high school mentally
SB - I assume you are looking to calculate pot odds when action is not closing? (i.e. chips left to play on the next street). If so, memorizing tables is not worthwhile. Implied odds, opponent tendencies @ next street sizing, how easily they can be scared off, etc - all of that matters more.
I only calculate pot odds on all in decisions. In which case, the only time the 2-4 shortcut doesn't work is on the flop, with a high number of outs. (Reason it is off is that you are double counting the times you hit the turn and the river). In most common situations (with play left), you should only be calculating the chance to hit the immediate next card - because we are expecting action on that next street. And then implied odds come in, etc. Almost always (lol V bet sizing) - you are getting the odds to continue in the situations you listed. Primarily because over pairs and sets and strong V hands offer great implied odds.