We've never been seated on time at Biergarten, so I would expect a 12:15 reservation to be seated around 12:20 or 12:25 (just my experience, it may be different for you). Since it's a buffet, just take your time working your way through, order some drinks (the beer sampler is a good buy if you're into beer or if you're an adventurous drinker) and you can easily still be there for the 1:15 show.