You choice. I'm just saying that if you wanted to add all at the same time, that's not a problem. You can add it slowly over the next week, that's fine too.

The 750 lbs is to get the salt level to 3000 ppm, starting from 0 ppm.
As you're a BBB user, you should check your salt level first, then add only what is needed.
There is no problem with oversalting the pool though, so I'm not too concerned with telling you to add 750 lbs. Higher salt actually feels better (to me).