I am using data on the opening, change and closing of short positions, but I am interested in when the profit/losses are made. Hence, I calculated daily the value of the short position by taking the cash gained with increasing the short position minus the current value of the shorts (liability). This results in a PnL, with both positive and negative values. However, now I try to calculate the return based on the PnL values. Problem is that the ordinary (new - old)/(old) won't be of much help when the old value is negative, for example an increase from -2 to +3, (3--2)/-2) = -2.5 while clearly this is an increase.
Does someone know how to handle data in such a situation? Moreover, I am new here, so sorry if I am asking the wrong question.