No, there currently isn’t a way to derive a value between records in the cohort definition framework.
Would it be possible for you to inject these ratio values into your measurement table at ETL time? Then you simply create a measurement criteria looking for that concept (the ratio of A/B) with the specified value (value as number = ratio between measurements, which you can look for a value greater than or equal to the specified ratio you seek).
The key is knowing when this ‘ratio’ occurred. If you take 2 different measurement records and calculate a ratio, what is the date the ratio occurred? The date when things occur in data is very important to the cohort building framework.