If the mixture spits onto the sides of the pan, use a wet pastry brush to wash the crystals off. Don't stir the boiling fudge too often - it lowers the cooking temperature and you are more likely to get grainy fudge. I tend to go by the colour when it's time to test - if th fudge has gone a medium golden colour it should be ready!Paler gold is Ok - you'll get a soft fudge, but if it's still pale yellowy it's not ready. Don't be tempted to beat the fudge too soon after you take it off the heat - it needs the cooling time to help form small crystals for the smoothness. You can add anything you like to the fudge, or just vanilla.