To answer your first question, to convert your running count to the true count, you would divide the running count by the amount of decks remaining to be dealt. So say you’re playing a six deck game and you have a running 12. If you look over in the discard tray and see 2 decks, you know there are 4 decks left to be dealt. So your true count would be 3. You would always round down. You can divide by half or even quarter decks if you want to but make sure you get accurate at whole deck divisors.
As for Spanish 21, I believe you would start the count by subtracting 4 PER DECK. So if you have a 4 deck game, you would start your running count at -16. Anybody feel free to correct me if I’m wrong