In stock form, it looks as if my front spring preload was a little low and the rear a little high. So too stiff at the rear and too loose at the front. I'm 175lb and 190lb with gear (I measure with gear).
I have the front down to 40mm, which is closer to where I want it. I usually go a bit firmer. But I'll ride it at 40 to see how I like it. 46mm to 40mm took only 1/2 turn, tightening the preload nut.
If I keep the front at 40, I'll set the rear at 44. That's around 30% of wheel travel.
The good news is that I won't need firmer springs. That's a pleasant surprise given that it's supposedly set up for a 145lb (65kg) rider.
Couple points, purely to help.....
Maybe just your terminology but preload (for all practical purposes) has no effect on the 'stiffness, firmness' of the fork or shock springs in a motorcycle. It only
affects ride height, nothing else. Probably THE most misunderstood suspension adjustment on a motorcycle. So misunderstood even manufacturers often get it wrong in their Owners Manuals. The description of its purpose in your supersport Owners Manual is technical correct, but vague. It just says preload should be adjusted for load. That is correct, to maintain the appropriate ride height, depending on the load.
As long as the preload adjusters have sufficient range, a correct rider sag (weight of bike and rider) can be set with any spring weight. The spring weight may not be ideal, but a spot on rider sag can still be set. To help understand this point, you could set a correct rider sag with an appropriate length block of timber substituted for the spring. But the spring weigh of the block of timber is clearly unlikely to be ideal.
The usual 'rule of thumb' to evaluate spring rates is to compare the relationship between 'rider sag' and 'free sag' (amount the suspension compresses from fully extended just by the weight of the bike alone). If you have set a correct rider sag on an incorrect spring weigh, it will show up in the 'free sag' measurement. For example our extreme stiff spring, the block of wood, the 'free sag' will be excessive (being the exact same measurement as 'rider sag').