For many of us it's loosely based on BMI. I know I chose mine because it's 10 pounds into the healthy range.
But from experience, I suggest aiming high.
I think 150 is your "healthy" number. If that's what you're aiming for, go for it and celebrate when you get there. I wish I'd chosen 145 as my goal because I'm feeling pretty good where I am, but since I chose 135 I stupidly have to get there. Dumb right? It depends on your personality, of course. My head's messing with me. But if you choose a high goal and reach it, you can always keep going. And you get to have a GOAL celebration along the way. Win-win.