UW says that "An opening pressure of >250 mm H2O on lumbar puncture confirms the diagnosis of IIH as long as secondary causes of increased ICP (eg, infection, tumor) are not evident on CSF analysis."
and that,
"After neuroimaging is performed, lumbar puncture demonstrating an elevated opening pressure (>250 mm H2O) confirms the diagnosis."
AMBOSS has nothing to say about the diagnostic value of a specific opening pressure for IIH, however, one of the sources that it cites in the table about CSF analysis (citation #13) states that:
"Normal opening pressure ranges from 10 to 100 mm H2O in young children, 60 to 200 mm H2O after eight years of age, and up to 250 mm H2O in obese patients. Intracranial hypotension is defined as an opening pressure of lessthan 60 mm H2O. This finding is rare except inpatients with a history of trauma causing a CSF leak, or whenever the patient has had a previous lumbar puncture. Opening pressures above 250 mm H2O are diagnostic of intracranial hypertension."
Source: UWorld - Step 2 107461