Are you saying that the interest rate moves in a 50% proportional ratio to the prime rate (for example, a 50 basis-point move in prime, or .5%, will reuslt in a .25% change in the interest rate)?
If so, how about something like:
The interest rate and corresponding annual percentage yield on your account are dependent on the Prime Rate as published in the Wall Street Journal. Your interest rate will change in direct proportion to 1/2 of any rate change in the corresponding index. We may change the rate on your account every time the Prime Rate as published by the Wall Street Journal changes.
You would have to also include any limits or caps on rate changes, including any ceiling or floors.
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The opinions expressed here are personal and do not represent opinions of my employer.