If you really want to be pedantic you could consult the graph from the datasheet and see what value it shows for the current you are going to use. But really it's a complete waste of time
It's VERY, VERY, VERY unimportant - a huge range of values will work - personally (if I was going to calculate a value at all) I'd just assume 2V drop, so 3V left and whatever current you were thinking of. I certainly wouldn't get out pencil and paper, or a calculator - 1K would give you 3mA, so 330 would give you 10mA - it's as easy as that.