Word for the range between two ends of a spectrum

Solution 1:

When two contradictory approaches seem to offer equally valid explanations to a phenomenon, the two approaches can be said to be held in tension.

The closest definition I've found in a dictionary is the following:

tension [noun]...

• c: a balance maintained in an artistic work between opposing forces or elements

[Merriam-Webster]

The doctrine of tensions is a well-known feature of at least many hermeneutical schools (eg the Bible teaches both predestination and free will). In the physical world, the wave-particle duality of radiant light etc is another illustration.

Solution 2:

sweet spot

An ideal or most favorable location, level, area, or combination of factors for a particular activity or purpose

The winery in Rutherford, best known for its loamy Cabernet Sauvignon, has a sweet spot of 8 acres for Chardonnay, which has produced spectacular wines. m-w

An optimum point or combination of factors or qualities.

Growth has settled into the 2 % to 3 % range that many experts consider the country's sweet spot.Lexico

These examples show that x-height follows the "Goldilocks principle"—the optimal x-height is neither too tall nor too short, but somewhere in between. Many typographic variables behave in this way; the "sweet spot" exist in the middle of the continuum. Karen Cheng; Designing Type

Leading is the amount of vertical space between lines. As with measure, if you have too little or too much leading, the text becomes hard to read. The sweet spot for leading tends to be 120%—145% of the font size. Yevgeniy Brikman; Hello, Startup