How do you figure 100 ft for every 10 ft of depth? At 7:1, it should be 70 ft for every 10 ft of depth (measured to your anchor roller or chock). So if your depth is 30 ft, and your anchor chock is maybe 5 ft above the waterline, you're talking about 7 * 35 = 245 ft of rode. (No tides to worry about on the lake, and I assume you don't plan to stay anchored long enough to worry about seasonal depth variations).
Also, keep in mind that the 7:1 rule assumes you plan to spend a while at anchor--like overnight, when you really don't want to drag. If you're actively fishing, then you're probably also keeping a close anchor watch, and you're probably not that close to other boats or shallows. If a storm comes, you'll probably weigh anchor and head home. You're in a position where dragging would be an inconvenience, rather than a catastrophe. You could probably get by with well less than 7:1.
I've done lunch with as little as 3:1 in my Bene 31 on Lake Lewisville (with a fortress anchor on a muddy bottom). I was nervous about it, but it held. And even if it dragged, it would have been no big deal to start the engine and reset it.
s/v Dancing Days
Lewisville Lake, TX