SAP QM - Defining Tolerance values for Quantitative Characteristics in percentage

I have defined a couple of Quantitative characteristics with units of measurement 'cm' and 'm2'. I want to define Tolerance values for these Characteristics as x% of Target Value. In MIC>>Quantitative Data, it is only possible to define fixed values for Tolerance. I also tried defining a Tolerance key with unit of measure '%' and then defining upper and lower tolerance limits in Tolerance Key. But his won't work as MIC and Tolerance Key has different unit of measurement.

Any inputs on how tolerance values can be calculated as % of Target Value?