How far to pressurise a saturation chamber on the rich mix before changing to the lean mix, so that it arrives at depth on the right PO2.
The formula: [(PO2 req - present) x 1000 - D x low%]/(rich% - low%). Change any box and the full working updates. For study only, never for dive planning.
The IMCA ADS, BDS and LST papers ask this as a four-option question, with a calculator on screen. A free account gives you 15 practice questions for each exam, each with the worked answer and the document it comes from.