Эта таинственная книга называется PHB.
И это базовое правило, а не опциональное.
Цитата
To determine how much time and money it takes to make an item, follow these steps.
1. Find the item’s price in Chapter 7: Equipment of this book or in the Dungeon Master’s Guide, or have the DM set the price for an item not otherwise described. Put the price in silver pieces (1 gp = 10 sp).
2. Find the DC from the table below, or have the DM set one.
3. Pay one-third of the item’s price for the cost of raw materials.
4. Make an appropriate Craft check representing one week’s work. If the check succeeds, multiply your check result by the DC. If the result * the DC equals the price of the item in sp, then you have completed the item. (If the result * the DC equals double or triple the price of the item in silver pieces, then you’ve completed the task in one-half or one-third of the time. Other multiples of the DC reduce the time in the same manner.) If the result * the DC doesn’t equal the price, then it represents the progress you’ve made this week. Record the result and make a new Craft check for the next week. Each week, you make more progress until your total reaches the price of the item in silver pieces.