The pre-load adjustment method should not change for different suspension units.
The Carby Varadero has a total rear suspension travel of 155mm.
The static sag should be 10-15% which is 15.5mm to 23.25mm
The dynamic sag should be 30% which is 46.5mm
To measure and/or adjust these values you will need a centre stand (or jack) and a friend to help you
Place the bike on the centre stand or jack it up so that the back wheel is off the ground. Record dimension A as per the attached image
Lower the bike so both wheels are on the ground and the bike is upright with no one sitting on it. Record dimension B as per the attached image
Sit on the bike with the bike upright and both feet off the ground . Record dimension C as per the attached image
Static sag = A - B
Dynamic sag = A - C
Adjust the pre-load accordingly if the values are incorrect.