You can't actually convert from one octane number to the other unless you can test the fuel. You can approximate the other octane number though, and 6-7 points is a reasonable estimate of the difference.

Euro rating is based on the Research Octane Number (RON).
Another standard is the Motor Octane Number. This uses a different test protocol than the RON method. One focuses on higher engine speeds, the other on lower engine speeds, as I recall. Edit: I looked it up, and the RON test is run at 600 rpm, while the MON test is run at 900 rpm. Since we don't spend much time with our road vehicles at those engine speeds, it shows the theoretical nature of the octane ratings.
North American Antiknock Index (AKI) is calculated as (R+M)/2, so it is the average of the two tests. However, two fuels with the same RON rating may have different MON numbers, and so have resulting different AKI numbers.

All that said, 98 RON is pretty close to 93 AKI in the real world.

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