peak pressure

how can you determine what your peak pressure is and how to manipulate it?

Without some quite expensive test equipment you can't, although transducers built into spark plugs are getting much cheaper.

You cannot really 'manipulate it', as it is what it is and you can just try and optimise the timing, fueling, etc to get the best torque from the engine at every rpm point without running into detonation or other problems. The point of peak pressure will just follow from that.