When do I apply R / R-1 complement?

Question

Given the following question:

• What is the following is the 3-digit 15’s complement representation of -251₁₀?

How would I know if I should apply R's complement or R-1's complement?

• 15’s complement of base 15 = (15n)₁₀ – N
• 15’s complement of hexadecimal number = {(16n)₁₀-1)} – N

The same question extends to other bases such as 9, would I use 9^th complement or 10-1^th complement?