You need to know where are you now, and where you want to be
The "demonstrated value" of Dr Anthony Grant is a great improvement on ROI because it looks at outcomes in two dimensions and allows explicit discussion of the business benefits of both. It's a model that can be used right at the start of the coaching in order for all parties to understand their goals within this two-dimensional framework.
People can come to coaching from all the boxes. They may appear to be flourishing (ie flying) in their current role, but be drifting in relation to their potential / long term aspirations. Or they may be successful, but feel unhappy. Or they can suddenly find themselves drowning during a major transition - struggling in the new role AND experiencing the distress of feeling incompetent.
Doing well can be measured subjectively by rating on a simple 1-10 scale, or objectively (over a longer period) by looking at relevant performance measures.
Feeling good can also be measured subjectively by rating your sense of well-being (which is about energy, confidence, optimism, happiness etc) on a 1-10 scale, or objectively using tools from the world of mental health.
"Success is also subjective" Dr Anthony Grant