According to my math and physics profs, the student who got the right answer the wrong way would get 100% credit, the student who didn't would get 0% if it's multiple choice (often with those common math errors from years past intentionally given as potential options), and maybe 50% or so if it was a long form answer and the class was merciful. For those long form answers, we would be given explanations on exactly how they would be graded by the lecturer and later the TA in recitation before the exam. For ochem synthesis problems, the prof would give you credit if you reached the correct intermediates at least, even if you couldn't solve the entire reaction, or the TA would take a point off for every mistake, and if the question was weighted for 8 points and you forgot 9 things, you'd get a fair zero.
These aren't shades of gray, they are just protocols you need to establish and stick with before these questions come up during grading. Then you need to stick with these protocols as department policy for decades, so your department is able to maintain the grading scale no matter who the faculty will be 50 years from now. That's the sign of a good department.