Learn Russian: Мы забыли о толщине стены в девяноста восьми сантиметрах от угла. - We forgot about the thickness of the wall at ninety-eight centimeters from the corner.

The phrase describes a location, so it uses the prepositional case after the preposition в. For compound numbers, every single word must decline. The number 90 (девяносто) becomes девяноста in all oblique cases, and 8 (восемь) becomes восьми in the prepositional case.

If 98 usually takes the genitive plural, why is the word for centimeters in the prepositional plural (сантиметрах) instead of the genitive plural (сантиметров)?

When a compound number is in the nominative or accusative case, the noun follows special counting rules (like taking the genitive plural). However, when the number is in an oblique case like the prepositional case here, those counting rules disappear. The number and the noun must simply agree in case. Therefore, both the number and the noun take the prepositional plural.

Does the -а ending on девяноста mean it is acting like a feminine noun?

No, the numbers 40, 90, and 100 have a special, simplified declension pattern. They only have two forms: they keep their dictionary forms (сорок, девяносто, сто) for the nominative and accusative cases, and they take an -а ending (сорока, девяноста, ста) for all other cases, including the prepositional case used here.

Why use the preposition в to mean "at" a specific distance?