True..........a sugar thermometer is at best hopelessly inaccurate, I use one as a guide only and a bowl of water for the definative result. See my rather long winded post on Baker's fondant for how to find soft ball stage if you don't know.
Here's a couple of recipes for fudge I used to make when I sold at shows. I made 6lbs at a time and it never burnt.
Known as Aunty Anne's
1/2 pint milk
1lb 12 oz sugar, brown and white mixed
4oz butter
vanilla essence
Bring milk to boil, add sugar and butter, stir till dissolved. Re boil and take to 240f.....soft ball. Remove from heat add vanilla essence, stir in and then stand pan in a sink of cold water, stir till it thickens then start to beat, it goes creamy. Pour into a 7" buttered tin (lined with silicone paper if you have it) before it gets too stiff to handle, smooth with a flat bladed knife. It makes a lovely grainy crumbly fudge unlike the modern smooth stuff.
Scottish Tablet
1lb granulated sugar
1/4 pint single cream
pinch salt
vanilla
Put sugar cream and salt in a pan over a low heat and stil till dissolved. Raise heat and constantly stirring heat to 245....firm ball. Take off heat still in vanilla. stil and beat as above and finally pour into a greased pan before it is too stiff.
Tablet is much beloved by anyone of Scottish descent or anyone with a granny with a penchant for sweet making. It was often made with a can of condensed milk instead of cream.
Frisbee