Seems like a lot of people are getting caught up in the details. It's probably impossible to calculate the future, but that is beside the point.
What the OP is saying is that, if everything in the entire universe follows the same set of laws, then its entire history was set in stone from the beginning. Free will doesn't exist, and the entire universe is clockwork toy marching toward heat death.
Possibly depressing, but I have a way around it: At some point in the past I came to the conclusion that philosophy is bullshit. The reason philosophical debates cannot be resolved is that they are meaningless, due their topics being concepts which were invented by humans and which have no basis in reality.
I learned later that that this position is actually a philosophical stance known as logical positivism, which came about as a rejection of the intellectual wanking which comprises most of philosophy. Logical positivism holds that for any concept or statement to be meaningful, it must be possible to test it.
In the real world, the concept of free will is meaningless. Whether or not it exists has no effect on our reasons for doing what we do, nor does it change whether our actions are meaningful.
Don't get your hopes up though, because it's increasingly likely that 'our' actions aren't even ours.
The human mind is one of the most heavily studied systems in existence, and quite a bit of research points to it being basically a mess -- a piecemeal collection of cludges and hacks to turn sensory data into information which allows us to interact with our environment, whether or not that information actually represents reality. Often it doesn't, and we never even know; look up inattentional blindness sometime.
Put someone in an fMRI and have them make a decision. It is possible to see what the subject will decide before they are even aware of it; sometimes as much as ten seconds before.
So forget free will, because measuring actual reality gives us plenty of terrifying questions to ask already. Does the person behind our eyes, which we perceive as our 'self,' actually do anything? Or are decisions made in parts of the brain we aren't consciously aware of, and only afterward does the consciousness find out about it and trick us into believing that 'we' made the decision?