Harrington's Zone Concept
The Zone Theory presented by Harrington in his classic book "Harrington On Hold'em" identified inflection points where your play in a tournament needs to change gears.
Dealing with limpers
Whenever a player limps in front of you, it destroys your chances of going all in to pick up the blinds with weaker-than-average hands. If you've got a strong hand, you can still go all in, but with a weak hand, you have to bluff out more players and that really hurts your chances.
The 3-Bet Jam Range
Typically, your prime 3-bet value is when you have 4-5 times raiser's raise value. If a player on the button raises to 300 and you've got about 1200, you're in a good position to reraise. It's too big for your opponent to call with weak hands but you're not risking too much to pick up the blinds and the raise. You typically want to start looking for three-bet jams when you're between 6 and 14 big blinds, as this will closely align with the 4-5x range.