OP, this is a good question.
Eight hours in a day is an arbitrary number. It may be stated explicitly in your contract (which, I'm sure, your employer will conveniently forget about when he would like you to work more than that much,) but this is still arbitrary. Eight is just a number. It could be seven, it could be nine.
Take, as an example, the recommendation, commonly held, that you should drink eight glasses of water a day. As it turns out, this is based on the recommended water consumption for a 35 year old man weighing 250 pounds. The eight glasses of water is based on certain assumptions which may or may not be true in your case.
I feel there are two questions here - is it reasonable to work a flexible amount of hours, or otherwise ask for other special provisions that might not be considered standard, and is it reasonable to expect your manager to understand and cooperate this.
The answer to these questions is yes and no, respectively.
Do I Have To Work Eight Hours?
No, of course not. As stated above, this is just an arbitrary number. It is based on assumptions about how long a person can be productive, and how much time a person needs for their personal life, that may not necessarily be true for you.
You are probably the only person who knows for sure how many hours you should work. Personally, I am not very productive after six or more hours of working. When someone tells me I should be productive for eight, I ask them where did you get that number from, and what assumptions are you making?
CEO's and other execs are notorious for leaving at three, or otherwise setting their own hours. This is not because they are lazy - most CEO's are very hard working. However, since they have no one to answer to, they manage their day based on their needs, versus a roughshod benchmark, meant to apply to everyone.
Is it reasonable to assume there is a way to convince my boss to see things this way?
It is certainly not a reasonable assumption to make. Your boss is a human being, and it is therefore completely impossible to ever know, for sure, how he will react to something or how to get him to behave in a certain way.
Typically, the reason a boss will operate under a quota-based system of hours, is because they are confused and scared. Constantly existing in a state of uncertainty about their job security (because they are not very good managers, those that assume all employees require the same thing) they try to exercise as much control as possible over the situation.
People that are confused and scared are not reasonable. If you offer a person running for their lives from a stampede of cattle $500, he will probably run right by you. This is because such a state of mind makes us very resistant to outside input.
What Should I Do?
In my opinion, you should not do something that you think doesn't make sense, even if your boss tells you to.
It's important to independently evaluate everything that you are asked to do, and make sure it makes sense, before you do it.
I say this because to do so builds critical thinking, and independence. When you do something you think doesn't make sense, just to appease a manager, you build co-dependence and complacency.
At the end of the day, if you spend years of your life acting in a complacent manner, you will become complacent. And it's your life, so it's up to you.
If your boss is not reasonable, you should quit. Right away. As they say, "the person who lost their job for a dumb reason, always finds a better one fast."