HI... the industry rule for assessing this is called SNAME 5-5. “Guidelines for Site Specific Assessment of Mobile Jack-up Units” and gives the calculations required for assessment.
Used to be that the rig contractors had to perform this study work for insurance purposes prior to moving to certain locations Study work then performed by specialist consultants feeding in geomechanics and metocean to verify rig suitability at a location. You might be able to get more information from them. The consultant will for sure have a detailed database of spudcan shape and spacing and can go onto analyse if rig can remain stable in required operating/metocean conditions.
The amount of leg penetration is dependent on soil properties, vertical reaction of the legs, and footing area. The greater the footing area, the lower the penetration under same circumstances. (PA: I cant image rigs are the same or soil will be the same each time a rig is taken there! This must be accounted for)
The amount of soil penetration should be checked against the footing structural capabilities and scour characteristics of the site. (PA: one surely needs a geo-mechanical analysis of shallow soils to conclude this?)
Proper planning with regards to soil information and predicted penetration curves should be done. (This is important to be done each time so plan plan plan!)
If during actual preloading the leg penetrations are recorded (penetrations vs. footing reaction curves), then this information can be used to improve upon the prediction penetrations curves methodology. (PA: Again it is important to assure this is properly doen and implemented each time?)