| Harv there is a rough rule for spring choice based around loaded and unloaded sag, which will be dependent on riders weight and bike setup, such as length between s/arm pivot and rear axle.
On a standard 748/916 I would say a 7.5kg/mm spring is to hard for a 10 1/2 stone rider. I think you should be looking at a 6.5kg/mm spring and get the rear ride height checked, ideally with a ride height tool if none available Section8 Superbikes have some good info on there website on how to do it from scratch. | 
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