Regular is 87 AKI*, High Octane is 92 AKI.
If your owner's manual suggests 91 RON** (87 AKI), just stick to regular. If it suggests 95 RON, for example, you should mix regular:HO in a 40:60 ratio.
*Anti-knock Index. It is calculated according to the formula (RON+MON)/2
**Research Octane Number. This is more commonly used around the world.