How much effort should one put into a business plan then (once the idea is solidified in their head)? Where is the point of diminishing returns? Is it still wor

How much effort should one put into a business plan then (once the idea is solidified in their head)? Where is the point of diminishing returns? Is it still worthwhile to do a five year forecast (especially when it's highly unlikely things will go as you plan (for better or worse) over that much time)?