I use my homegrown hops a LOT for bittering. Like almost every batch. You can learn through trial and error what your alpha acid is. It varies a little from year to year but usually is pretty close depending on your weather and consistency of harvest time.
I can't say I have used Nugget for bittering yet, but if memory serves, I thought it was a low cohumulone hop, so it should be just fine for bittering. Like hopfenundmalz said earlier, use the average (13%) your first time with them and then adjust from there if you're a little off. It probably will be almost perfect on the first try, assuming you didn't harvest way too early or something like that.
My homegrown Cascades have averaged 6.1% since 2009, with a range from about 5.9 to 6.2%. It hasn't varied a lot. These values are approximate, based on how my beers taste vs. expectations. YMMV.
<soapbox> Magnum is great, but for heavensakes, there's other bittering hops out there. Expand your horizons. Maybe even try some homegrowns! </soapbox>