There is no 'magic number' for this. And frankly, sometimes the investment you make with products to ensure consistent growth is more outside development than inside. For example, one of our customers did a win/loss analysis of a mature market in an attempt to understand and stem the tide of license cancellation. For little more than $60k, they were able to immediately implement systems they believe stopped $2 million in cancellations. (this was about 5% of annual revenue). But the changes they implemented were in personnel costs. They never opened the code.

As for the question, we know of no research or estimating method to determine how to allocate money between sustaining engineering and new development. In our Roadmapping seminar, we use an "ROI Evaluator" to show the allocations across a portfolio to the exec team for their decision-making. The purpose is to understand the portfolio in terms of overall ROI vs. Strategic Fit. So, if projects are out of alignment, they can be more closely scrutinized.

Answered by John Milburn