First Order Rate = (COUNT DISTINCT(Order ID) WHERE Customer Order Count = 1 AND Dimension = xxx) / (COUNT DISTINCT(Order ID) WHERE Customer Order Count = 1)

This KPI is contextual - it's the percentage of first orders that fulfill a certain requirement.

In context of a product for example, the First Order Rate will tell you what percentage of First Orders contain this product.

Since an order can contain multiple products, the sum of this value across all products is likely going to be larger than 100%.