Recommended pressures are usually listed on a sticker inside the driver's side door. If not there, then the owners manual will have a chart, and BMW helpfully shows a chart in iDrive for OEM tires for a specific vehicle. Nothing wrong with using the loaded pressure recommendation as a starting point (you'll get better mileage, but a potentially harsher ride), and adjust down from there - underinflation is worse than oveinflation with respect to tire life, fuel economy, braking or acceleration. That's what I use, because sometimes the car is loaded, sometimes it's not, and my label doesn't even have a partially-loaded pressure recommendation:
Just never use the maximum inflation pressure listed on the sidewall as a guide - you're in dangerous territory if you're at or above that.