I think it boils (sorry!) down to the blend of coffee, personal taste and simple trial and error! The guidelines on the coffee packet are just that. I've never succeeded in finding a good tasting coffee or being able to make a 'perfect cup'. If you find out the answer, let me know!
It depends how many cups/mugs coffee you want to make! I'm a coffee lover and VERY particular about its taste. Sainsbury's Italian is the best I've tried. NEVER use boiling water to make it and measure your water by the mug(s) you're using.To one mugful of water I use 3 heaped teaspoonsful of coffee - I like mine strong and black (like my men!!!) and then I let it stand for about 4 mins before pouring.