Since you feel that you are underpaid and your salary is 30% lower than median, and assuming that you want your salary to be 20% above median, just tell them th

Since you feel that you are underpaid and your salary is 30% lower than median, and assuming that you want your salary to be 20% above median, just tell them that "the salary I'm looking for is around 1.7*X" (where X is your current salary).