Could I make this by steeping saffron threads instead of using ground saffron?

Also, you could toast your threads lightly in a dry skillet and grind them with a mortar and pestle, but that does sound like a formidable lot of saffron.

It's not a silly question at all. Ground saffron is a common shortcut in Persian and Indian kitchens- but even they are really picky about the company that produces it because it can be so easily adulterated with other reddish spices- paprika and tumeric just to name a few. The amount you use depends on how deep you wish to have the color, and how pronounced you want the flavor to be. Certainly you can use a good size pinch of your threads and steep those in hot liquid instead of using the ground product. It was done that way for a long time, and still done that way by some. Threads are also more likely to be flavorful, since grinding shortens the shelf life. So go ahead with that plan. If your saffron threads are particularly fresh and strong, maybe a small pinch- if not, a larger pinch. Your nose and eyes should tell you what you need to know as it steeps. Keep in mind that strong saffron flavor can also be a bit bitter in a finished dish, too. I don't buy ground saffron, but I usually get my spices from The Spice House, which has always been reliable for me. But there are plenty of others online and you can also get it at some Middle Eastern food markets.