How to measure a cup of “solid” ingredient

I've always been taught that baking is a science when it is compared to cooking. Cooking is very much 'to taste' and very individual. There are not as many things that can go wrong with a standard recipe in cooking, and you have a lot more room for creativity. You don't have to look at baking as that precise. However, unlike cooking, where you can add or subtract from a recipe with no real harm to physical structure, that does not hold in baking. For examlple, if you feel that there is too much salt in a recipe, cutting back can (and most likely will) have a cascading effect through the ingredient chain. You have to understand your ingredients and the effect they have on other ingredients. That is what makes baking a more precise science.
And in terms of flour, it is often the most 'ranged' ingredient. Depending on flour type, miller, altitude, water temp. etc., the amount of flour in a given recipe is always a guide. Again, you have to know your ingredients. You will never see 'one and an eight cup plus 2 TBSP hard flour' in a recipe, because it is so variable for many different reasons. Flour and water are the two most flexible ingredients, and are always variable. Hopefully this helps, I am sure someone will come along with a more scientific explanation for all different ingredients, I am just offering an experience based answer to your question. Also, you are right, do not mess around with baking soda and powder. If you do, you are asking for trouble. I would suppose that the rule of thumb would be that the more exacting an ingredient is (tsp, quarter tsp), you don't want to change much.

This is something that really puzzled me when I went living in an English-speaking country. Having grown up in Italy, where recipes tell you "Put 120g of this then add 250g of that", I've always used a scale. The first time I saw I recipe with cups and teaspoons I said: yeah, but I've got 10 types of cups in the kitchen... which one should I be using?

Then someone told me that they were real unit of measure and introduced me to measuring spoons (something that in Italy would be very difficult to come by).

So, going back to your question:

1. I strongly second @Adam's opinion of buying a digital scale: they're cheap and do the job (personally, I would not spend more than $30-40 on it, but you can find $300 ones if you wish).

2. If your recipe calls for teaspoons, cups, etc. then you can:

   • Buy measuring spoons/cups (you can find a complete set for $10, for instance this)

   • Convert them to g (or whatever your scale measures) using an online calculator. If you google for "teaspoons to grams" or "cups to grams" you'll find plenty.

You can even get an app on your phone. I use this app which works pretty well (I'm sure there's a similar thing for iPhone, but I don't use Apple products, so I can't help there).