In established, competitive markets, if a brand increases its share of advertising spend it is likely to grow (and if it decreases its share it is likely to decline).
The impact of spend changes on market share varies a lot by market so the best way of estimating effects is to use historic data. Sales modelling using sophisticated statistics gives the most accurate estimates but even a simple cross plot like the one shown here can be used to provide a rough approximation.
The line of best fit can be used to determine the change in spend typically needed in the market to deliver the desired change in market share.
Listen to this audio clip to hear more about the topic.