If a combine harvester allows a farmer to feed as many people as 100 farmers working by hand then they have a force multiplier of 100x. If you can train one nurse to deliver as many babies as 10 normal nurses then they have a force-multiplier of 10x. This doesn't necessarily translate into 10x or 100x the salary - farming is still an extremely capital-intensive business, probably moreso than it was pre-Green Revolution. Tractors and fuel and fertilizer don't buy themselves. There really hasn't been a single improvement that produced a 10x improvement in the number of babies a nurse can deliver. Maybe if someone goes out and invents the robo-midwife, but I can't see that being real popular...

The other way you can go is a qualitative improvement, which is where the child mortality example comes in. If one engineer can remove 75% of the fuck-ups caused by manually performing a task, then that is also worth money, but it's not valued in quite the same way. You'd value lower child-mortality in person-years of economic output (accrued to society), quality-adjusted-life-years (accrued to the individual) and diminished malpractice suits/insurance (accrued to the hospital or the insurance company).

Perhaps a better term would be "productivity multiplier". Productivity is a pretty standard economic measure (average economic output per individual), and engineers both have much higher economic output per individual, and can greatly increase that output for other individuals as well (often with a one-time investment).