I don’t have my copy of TCB with me, but I’m going to do my best to try to help you here.
First, let me refer you to some tables near the back of the book (I don’t know the exact page number, but near the section discussing wedding cakes). You will find one page that has a table of information based on the size of cake you are baking with number of servings per two layers of the given size, an appropriate Rose Factor (RF) to apply to the base formula, which level of baking powder to use, as well as the appropriate mass of batter in the pan. In the table, as you go down the far left vertical column of information you will find pan sizes. Find the row of information for a 12” cake and then as you go across the table of information from left to right you will the appropriate RF, baking powder level, # of servings for two layers of that cake size, and mass of batter to pour in the pan. Once you have the information for a 12” cake you can apply it to the base formula found on the following pages.
Next, find the base formula for the chocolate butter cake (each cake type: white, yellow, and chocolate have their own base formula). In this table you will find what Rose refers to as the “base formula” with the quantities of each ingredient to use. These quantities are what you will apply the RF to multiply each ingredient by. Since I don’t have my book with me, I’m going to use some numbers as an example, but these aren’t necessarily the numbers in the actual base formula. You’ll have to find the correct numbers in the book. Plus, I don’t think we’re supposed to print the actual numbers here anyway due to copyright rules and general respect for Rose.
Remember, I’m making up these numbers I’m about to use for the purpose of this explanation. Check for the actual numbers in the book. Now, suppose the table of information tells you to use an RF of 4 and a baking powder level of 3. Furthermore, suppose the base formula tells you to the base amount of cake flour is 25 grams. This would mean you multiply the base number (25) by the RF (4) to give you 100 grams of cake flour to make a 12” cake. You repeat this process for each ingredient in the cake and multiply each of those by the RF as well. So, if you had 10g sugar in the base formula, you would multiply that by 4 and use 40 grams. Also, it’s worth noting that in some cases you can choose between different RF values to use. For example, if you have a choice of using 3 or 4 as your RF, it’s up to you which one you want to use. Just be sure that once you make that choice you use that one to apply to all of the ingredients. You wouldn’t want to use RF 3 for some of the ingredients and RF 4 for others.
Now, for the baking powder there is a separate table of information (separate from the information in the base formulas and usually right below the base formula table) about how much baking powder is used according to which level you are using. Using the numbers from my example (again, they will probably be different than what’s actually in TCB since I’m just making up numbers for this explanation). The level of baking powder you use for any given size of cake can be found in the table of information I told you about in the first part of this post that has cake pan size, BP level, RF, et cetera. Now, suppose the table tells you to use level 3 baking powder for a 12” cake, then find the amount of baking powder used for level 3. You then have to multiply this amount by the RF, too. So, suppose level 3 tells you that it’s 3 grams of BP and you’re using an RF of 4, then you would have to multiply that 3 grams by 4 for a total of 12 grams of baking powder. Also, I believe each cake type (white, yellow, or chocolate) has its own separate table of BP levels, so be sure to use the correct base formula, as well as the correct table of BP levels.
Once you have your batter prepared you just follow the guideline from the first table so you know what mass of batter should be in each pan and then bake at the appropriate temperature and time.
One last thing to mention here. A while back I think someone told me that there was a typo in the information in the first table with RF, BP level, batter mass, et cetera. I think it was regarding the serving portion amounts, or how many layers the formula was for. I believe that applying the RF to the base formula (for any given cake pan size) is to make one pan of batter (i.e. one layer), even though the table provides information regarding how many servings are made for two layers. Hopefully someone will chime in here to provide clarification on this matter.
Anyway, I hope this helps. I don’t know how soon you plan on making this cake, but I will be heading home in a few hours and then I’ll have my copy of TCB in front of me, so if you need to, you are welcome to send me a PM and we can discuss the numbers a little more in detail if you need to without actually publishing them in the open. Good luck and have fun with this, chocoholic!