When a recipe calls for a cup of chopped nuts, should they be measured before chopping or after chopping

I would do it the other way round, I'd fry the sausages first, then add the veg. This has a few benefits as I see it:-

• The sausages will brown more evenly, purely aesthetic but some people will think they are not cooked if they are not brown.
• You'll get the oil out of the sausages so you'll have a better idea of how much oil to add when you add the veg, if any.
• The veg will sweat down in the sausage oil which is likely to be much nicer than the other oil
• You can cook the sausages on a really low heat to start with so that they really get sticky and caramelised, then when you add the veg back in you can add a spoon of water/vinegar/stock to deglaze the pan an get that lovely sticky sausage goodness all over your veggies, which will be delicious.

You could optionally take the cooked sausages out of the pan and do the veg separately, or keep 'em in there, I'd probably take them out so I could turn the heat up a bit without having to worry about the sausages.

I'd say that when you do add the veg you want a thin (1mm ish) layer of oil in the pan. Depending on the pan this is probably a tables spoon or 2. You probably don't need to scale up as if you are cooking a bigger batch you will probably use a bigger pan, and so a similar depth of oil should suffice. If you are using the same pan, but with a deeper amount of veg, then I would add a little more, but I would make the depth of oil in the pan proportional to the depth of veggies.

Remember the golden rule. You can always add, you can never take away. And once the veg start to cook down and the volume decreases then the oil ratio will be higher, so start out with a little less than you think, just enough to coat the veg when tossed in it.

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.