Currency Exchage Rate - Currency Spread

If NZD: USD = 0.6000 - 0.6010. For calculating percentage spread on USD do we need to calculate USD: NZD bid ask quotes from the given NZD:USD quote and then calculate the percentage spread on USD? My question is when they ask us to calculate percentage spread on a specific currency, say US , does the bid-ask quote that we should use for calculating the spread need to have US as the base currency? In Schweser, Reading # 18 ( Economics) Page 321, Question 6, they ask us to calculate the spread on USD…don’t we need to use a bid- ask spread that has USD as the base currency?