It can range on the size of the lemon and the time of year. A medium lemon will give 2-3 Tablespoons of juice, where a larger lemon can give 1/4 cup (4 Tablespoons). Just have to decide on much lemon flavor you want in your recipe as to which number you pick.
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.
Best Answer
If the punch recipe is 1/8 frozen juice, 1/8 fruit cocktail etc. then this is to make scaling easier. You'd use equal quantities of all ingredients that are each 1/8, and twice as much of anything that says 1/4
Example: If you wanted to use a whole 12 fl. oz container of juice, you'd need 12fl. oz of fruit cocktail, so you'd have a bit left over. You'd make 8x12=96fl. oz in total . If you wanted to use the whole container of fruit cocktail, you'd need to open a second container of juice. In this sort of recipe though, the difference between 12oz and 15oz is minimal, so you could easily use the whole container of both.