Is it like Earth ? The radius of the planet itself will have a bearing on how much radians (degrees) cover 800 miles.
Also, did you consider that if the top is so high it can be seen from 800 miles, it might not be possible to actualy see it from... 10 miles ? From that distance it would just go up up up and disappear into a point. (Or so I'd imagine).
I can't say much about the first question, but I believe I have the answer in hand. For the second, yes, you're absolutely right. This clocktower is designed with smaller and smaller faces running down its sides, so that you can still see the time even when you're too close to see the top.
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