Nice questions on stack-to-pot ratio!
It's always calculated with the effective stack. For example, if the pot going into the flop is 500; you have 10k and your opponent in a heads-up spot has 1k; the SPR is 2.
If the spot above was multiway and there was another opponent with a stack of 2k, then it would make sense to talk about having an SPR of 4 with this player and an SPR of 2 effective with the initial player.
The concept is a very useful one but you definitely don't need it in every hand. The idea is just to let it guide you generally in making decisions like to stack off with a weak top pair because the SPR is low.
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