For big stouts like a Russian Imperial, is 15% specialty malts typical? How is the percentage of specialty grains determined, and how does that percentage get broken down into individual components? Is there a typical format to follow in general?
For example here you mention a specialty grain breakdown below with Special B and English Medium Crystal tossed in:
Is it more by gaining experience by brewing and researching other recipes? In other words you have feel for it now, and intuitively can concoct something that you know will be good, or is there some general formula combined with experience?
Edit: Before I forget to say this. Before I truly start digging into making a recipe, I read, re-read, read it again, and then read it about 10 more times the BJCP style guidelines for the style I want to brew. Not that you have to follow BJCP guidelines to brew good beer (not at all in fact), but if you have a target to shoot for, you can hit much closer to the mark. When I started reading this thread first thing I did was go to my BJCP style guidelines app on my phone and paid special attention to the aroma and flavor descriptors.
To some, 15% would be a tad high even for a RIS. There is the risk of complexity turning into muddled flavor. Think of it like a dish of food you are cooking for breakfast/lunch/dinner. If I am making some overeasy eggs and toast with bacon I'll throw a pinch of kosher salt and fresh black pepper on the eggs, its also good with a dash of hot sauce, and then a dash of toasted ground cumin, and then a spoon full of salsa... so on and so forth pretty soon its not eggs and toast and bacon, all you taste is the salsa and the hot sauce and theres some egg chunks mixed in. IPAs are the eggs and toast, RIS is the huevos rancheros, it can handle a bit more specialty malts and still taste like a big roasty RIS(you won't be able to taste all the invidual contributions from the specialty malts, you never really taste bay leaf in soup, but all soups are better with bay leaf).
Before I get into your second set of questions. Nothing can replace experience, as I'll talk about below. I wish I had tried other "tried and true" recipes when I started brewing. Granted I got to learn a LOT about different specialty malts and what I like and don't like and what they do in different amounts.
As for your second set of questions. Its a little bit of everything. I use my culinary experience to make sense of a beer recipe. It took me writing and brewing probably first 10 batches were bad-onlysomewhatbad, then the next 10 recipes were mediocre-somewhatok. My past 5 batches and a beer I made back in March were really awesome, one of them won a county fair and is being brewed at a local brewery as a pilot batch. I went the dumb way and just started writing recipes and then brewing them to find out what does what.
Some beers, for example a witbier, theres really not much you can do to improve it. You can play with it, but the general formula for a good witbier is largely the same. Part of it is the region the style comes from you are trying to emulate. Belgian beers for example are a two to three note "song" so to speak, pilsner malt, sugar, and *maybe* a small amount of wheat. Stouts usually are 4-7 note "song", base malt, adjuncts like oats, primary roast character malt, then you have a few roast/chocolate/bready/malty/caramelly malts to build up the back ground to support the primary roast character.