Ideally, you want 7:1. In real life, you have to work with the conditions available. The longer the scope, the more opportunity for trouble due to swinging at anchor. I've been awakened in the wee hours by a boat banging into mine because the guy thought lots of scope was a good thing. I was a Boatswain's Mate in the USCG at Station Cape Disappointment. Part of the motor lifeboat coxswain school was the CG's study on anchor types and anchoring. For boats, the best holding power came from 3-strand nylon with about 6' of heavy chain attached to a Danforth-type anchor. The chain holds the anchor's shank down, improving its angle of attack, and it protects the nylon line from abrasion. The heavier the chain, the better. We practiced by anchoring a 44' MLB in the surf. Lots of fun, that. Later, I bought a 12 ton Hillyard motorsailer. It had chain, a plow anchor, and a big, heavy winch, which is what you want if you're going to stay anchored for a while. I got rid of all that, freeing up the foredeck and getting a lot of weight out of the bow. I had 100' of 1" dia. 3-strand, 6' of chain that weighed about 20 pounds, and a 25 pound Danforth. I had a retriever line and float attached to the anchor. Pulling the anchor out by its base is much easier than pulling it up by its shank, and you always know where the anchor is. We cruised the inside Passage a bunch, and never once did we have the anchor slip. We anchored in 30' with a 100' of line and 6' of chain frequently. The cool part was that the water was crystal clear and you could see what the anchor was doing. I'd get up in the morning and find the anchor had dug down out of sight with half the chain showing. The really big disadvantage of the rig is marine life (icky slime and barnacles) loves nylon line. It's for short-term use. I never try to set the anchor. It just plows up the bottom. The anchor will set itself. You can put your hand on the anchor line and feel if it's slipping even slightly. Know that the bottom is composed of so you can avoid poor conditions.