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Step size with equation in staircase on builder

Hi all, how should I enter the step size for staircase in builder if the size is not a fixed value, but a linear equation (of midpoint)? Is the builder able to enter equation or I need to change in the coder?

Here is my step size in detail
I am trying to make a decision-making game between two gambles by using staircase method. The max and min value are (5,30) Basically, subjects will be shown with 2 gambles: 50/50 chance of winning $5/$30 vs. certainty of winning $15. If subjects choose the 50/50 gamble, the amount of certainty money will be increased by the equation (15+max)/2= $22.5 at the next trail; if subjects choose the certainty $30, on the next trail the certainty money will be decreased by the equation (15+min)/2=$10. The same rule continues until the 6th trail.

Thank you so much!!!