The 2.5x BB raise - why??

I raise whatever seems to be taking down the blinds. 2.5x frequently will take down the blinds.

I also raise 2.5, or even 2.0, at various points for pot control based on my opponents stack. Say he has 1000 in the BB and it's 50/100 blinds (start with 1500, Stars). Due to his chip/blind ratio, 2.5x is enough to generate fold equity. He might call a few hands he'd otherwise fold, but that's ok, because the real reason for my 2.5x bet is post flop equity.

If I raise preflop to 250 that leaves him with 750 in a 550 chip pot if he calls (assume SB folds). If I then bet 250 as a continuation bet, he's looking having about the same chips as are in the pot and that's important to his decision process.

He has enough chips to fold in a pot this size. That's the key. He can live to fight another day. It's a pot worth having...but perhaps not worth going broke over.

If I raised 3xBB preflop, post flop odds end up more tempting for him. Now he has 700 chips in a 650 pot. If I c-bet 300, now he's looking at a nearly 1000 chip pot and he may feel any-two cards are worth going broke with.

A 2.5BB preflop raise puts me in a situation to have good fold equity on a c-bet where a 3xBB preflop raise doesn't.

Also, in this case, if I'm betting more than 2.5BB, I'm betting probably 4-5BB, not 3. His chip count (10BB) is at an inflection point where he can play a small pot or an all-in. Which I bet depends on my hand. Hands that are hard to play post flop (22-77, A4s, etc) I'll bet 4-5BB preflop. I want to win or be so pot committed that it's and easy call if he shoves back.

Hands that are really strong (AQ+, 99+), I'll raise the 2.5BB because they're strong enough c-bet on many boards or call if he shoves.

Hands like 89s, JTs, etc are the tough ones to decide how to play for me.