Calculated line in a dot chart

We have a dot chart where the size of the Turnover per customer is shown on the X axis and the Discount-% percent is shown on the Y axis.

There are 2 dotted lines showing the average (Discount and Turnover).

I would like to have another horizontal line showing the 70% percentile on the Discount-%, but I can only type in a number not a calculation.

My request is the possibility to add a calution in the set up box - marked with yellow in the bottom