When trying to calculate the approximate constitution of as-cast tin containing aluminium alloys one has to cope with a combination of intricacies: (i) Scheil solidification simulation may reflect strong enrichment of alloying components, especially in multicomponent alloys, thus leaving the safe ground of the underlying thermodynamic database. (ii) Liquid demixing often intensifies by addition of many components to Al-Sn alloys, thus forming monotectic reactions, boosting the segregation and aggravating the first effect. (iii) Scheil simulation in multicomponent Al-x-y-z-Sn alloys not only combines the first two problems, moreover, the current versions of major thermodynamic software packages are not able to perform the Scheil simulation if liquid demixing and monotectic reactions occur. These intricacies are worked out and the development of a dedicated Al-Si-Cu-Mg-Sn thermodynamic database for large composition ranges is presented. Calculations are compared to experimental data of an Al-7.5Si-3.5Cu-0.3Mg-0.1Sn alloy and the need for specific follow-up work is identified.