If I currently sell 10,000 units, and my use of Formula 1 indicates that I will need to sell 500 additional units to justify my suggested change to the marketing mix, what percentage of sales does that represent

It represents a 5% change to the marketing mix.

Explanation:

The change = 500/10,000 x 100 = 5%.

Company A's change in a variable can be compared with another index, by expressing the change (addition) as a percentage of the index.  For instance, the sale of 10,000 units is an index.  The additional 500 units that is needed to be sold represent the change.  In percentage terms, the change can be divided by the index and then multiplied by 100.