Online tests with automated response systems are often used as pre-assessments or to give students formative feedback on their progress during a course. As an incentive, bonus points are commonly awarded. The pass threshold plays a crucial role: A low pass threshold encourages participation whereas a high pass threshold signals clear expectations for the course, yet is sometimes mitigated by allowing for repeated tries. In two first-year maths courses in different degree programs online tests with STACK questions were introduced. These tests are offered independent from the course format. They have a high pass threshold, allow only a limited number of trials and count towards the final mark, i. e. are considered a summative assessment for the course. However, to allow for practice and formative assessment, "sandbox" environments were implemented allowing for unlimited practice. This article will give an overview over the choice of parameters for different implementations and discuss the findings on the use of the "sandbox".