Generally speaking the belt usually rides on half or less of the circumference. compare old and new, get the difference and divide by half. subtract that from the belt and it should be close enough to work.
Otherwise, your current tensioner may be able to handle adjustemnt under 1/2 in,
Side note, most things spinning have an effective useable RPM range. A larger driving or smaller driven pulley may bring top RPM out of range at high engine RPM.
Side note, the required HP to spin anything will increase with a larger driving or smaller driven pulley.
When life throws you a curve......Drive Baby, Drive.
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