Stocking up makes no sense in Factorio, it's more about a steady flow of resources, keeping up with the demand. The stock will mean nothing if you can't keep it up.
You should keep a small buffer to compensate for fluctuations, but anything above that is actually just a frozen asset, that could be used more effectively elsewhere.
I don't want to be rude, just wanted to point that out. Have a good day fellow Engineer!
Building on this good advice, the appropriate size of that "small buffer" is a function of your usage rate and of how quickly you can respond to incidents: When your Uranium patch runs dry, how quickly can you tap another one, and how much nuclear fuel will be used from your buffer in that time?