Idiom for not not investing enough to get expected value

Consider a team scoping projects at work hoping to get partial value for partial work. They assume that accomplishing 80% of the work will derive 80% of the value of that work. Reasoning that the last 20% is really expensive but 80% is still worth the effort. In this case the last 20% is expensive, but there's a minimum threshold - say 90% of the work - that has to be completed to get any value at all. So they continue to invest 80% instead of the 90% and see very little return.

I'm trying to articulate this and argue that it would be better to invest in a few things to 90% than multiple things at 80% if we aren't getting value out of the bulk of the work we do.

Consider "close but no cigar". It can be used as a mild rebuke, almost light-hearted.

close but no cigar phrase (of an attempt) almost but not quite successful. ‘they didn't catch him in the car—close but no cigar’ -lexico

Reader's Digest suggests that the phrase comes from those seemingly-trivial but nearly-impossible competitions common in fairgrounds:

The expression, “Close, but no cigar” means that a person fell slightly short of a successful outcome and therefore gets no reward. ...

The phrase most likely originated in the 1920s when fairs, or carnivals, would hand out cigars as prizes. At that time, the games were targeted towards adults, not kids. Yes, even in the ’20s most carnival games were impossible to win which often lead the owner of the game to say, “Close, but no cigar” when the player failed to get enough rings around bottles or was just shy of hitting the target. As fairs started to travel around the United States, the saying spread and became well-known.

In your example, the team put effort in, but didn't do quite enough to be rewarded. You could also say that their efforts didn't pay off.

This sounds like the concept of the minimal working example. The concept is often used in the context of computer code to indicate the shortest code that can reproduce an important effect. More broadly, the term represents a functionality threshold: in your case, less than 90% effort doesn't yield functionality, so an important distinction exists between, say, 89% and 91% effort.

You could also say that 90% investment is the benchmark—the input required to bring the effort to life or for the project to come to life. A certain level of work is necessary to put you over the top. Upon reaching this point, you can call it a win; you can ship it.