I would personally say you want to be on a 156cm board, but you already have your stick so that's irrelevant now I suppose.
Setting back means putting your bindings further towards your tail than the nose.
If you used only the front holes, you'd be setting them forward, which would be very counter productive.
You want to set your bindings so to give equal distance from nose and tail. The only way you will find your preferred width/angles is by trying them.
Set your bindings to use the holes closest to the centre of the board, on both your nose and tail. This will put you dead centre. Then slowly move them out, at the same distance each, until you find your preferred stance width. So for example, rather than being 50.5cm away from the nose and 47.5cm away from the tail, you'd be 49cm away from both.
You will find your weight when riding will be over the centre of the board this way, rather than over your tail. This should counter any nose flapping issues that you've been having. Also in your riding technique, make sure you are keep your weight over the front of your board. You may find you have been leaning backwards somewhat, which people tend to do when they are learning.