Geoff, I rarely anchor in more than 10m and would use both my 5kg 'angels' at that sort of depth for an overnight stop (lowered to the warp/chain junction). Total ca.3x to 4x depth .... depending on conditions.
Most of my anchoring is in shallow water (depending on the tidal range) 5-8m or so. In which case I would put out about 4x the depth in brisk winds(maybe only 3x with 'angels')or if I needed to restrict swinging. In very shallow to drying water, never less than 15m altogether.
I have a feeling that the 'serious' anchoring pundits will tell you that you need a lot more ... but this tends to refer to much heavier boats and in deeper water.
None of this refers of course to survival conditions on an exposed lee shore. You need enough chain to lower your waterline significantly to cope with a storm!
But for 'normal' purposes the 'angels' make such a difference .. keeping the short heavy chain almost horizontal for much of the time.
They are also useful to take up the slack of the long lines needed to allow for tidal fall when drying alongside quay walls etc.
I made them from bits of scrap lead melted in an old aluminium saucepan on the barbecue .. plunging a spike down the middle just before cooling to make a hole for securing a warp.
Chris.