For storing hops, a moisture content of 8-10% is desired.
The best way to determine this in a home-based setting (and oft performed in the lab as well) is a weight by difference technique. This is a destructive technique (you won't be using these hops for brewing after).
Take a sample of your hops, weight them. Then put them in a toaster oven (or other similar device). The goal here is to dry the hops out to completion (removing all water). You'll want to use enough heat that they dry out, but not so much that they burn or otherwise degrade, it has been recommended 120-140 F is fine for this, but you'll want to use a lower temperature for your bulk drying. Check the weight of your hops regularly over the course of the drying, once the weight change levels out, they are dry.
You now know the moisture content, and the dry weight. Some simple arithmetic and you should be able to determine how much to dry your bulk hops to proper levels.
Here are two excellent articles that cover this topic for home-use in thorough detail, including the math!
1. Drying hops on a small-scale
2. Hop Drying at Home