When a server has say, 99.3% uptime, how is this calculated?

I'm writing an app to poll UO servers and I will include uptime. What I'm thinking of doing is this:

Poll each 5 minutes and insert an entry into a SQL DB (I need to do this for other data) if it's up, I write 1000 to the uptime field. If it's down, I write 0. When I go to calculate the uptime I will take the average of that field and divide it by 10 so it gives me 1 decimal precision.

Would this work out ok? If I want more precision I can easily make my number bigger as well. For this purpose 1 decimal should do, maybe 2.
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