ぬか is an old-fashioned reading of 額, which is commonly ひたい today. That information is pretty easy to find if you type in both 額 and ぬか in a search engine.
At the bottom I believe you're talking about 当て字(ateji), but that is not relevant to the 額 question.
Please, take a look at this related discussion in reddit, in particular to the following passage:
[...] As for the pronunciations of 描くI believe they are fairly interchangeable, but えがく is used more for artistic sketching or 'picturing' and かく for drawing diagrams.
The linked post from ConMan in the comments (Do we really need to remember the kunyomi and onyomi reading of each kanji?) goes most of the way to answering your questions. But, to directly answer them:
When do you use these readings? -- when the specific piece of vocabulary that is being used demands it. That is to say, you can't know for sure unless you ...