The crucial question is what tolerances shall be applied to the fixture dimensions to ensure correct tolerances on the part dimensions.
The literature is generous with suggestions. They range from one half to one tenth of the part tolerances.
Each recommendation is as good as any other recommendation, and none of them will guarantee the correct result.
Fixture tolerances are determined by a combination of common sense, practical judgment, logical conclusions, analysis, and calculation, usually of an elementary, sometimes trivial nature.