This is an easy question, I hope.
Suppose we have a swap A with a long position, which, originally, has a weight of 30%. Over time, it has a positive performance of 3%, meaning we have a multiplier of 1.03 relative to its original value. Hence the resulting weight would be 30% * 1.03 = 30.9% (as a % over original portfolio value). This makes sense, because you expect the value of the holding in A to increase as its performance increases, hence so will the weight.
If we now have a swap B with a short position with an original weight of -20%, and the swap performance is -2% (0.98 relative to original value), we would expect the portfolio to perform better as you benefit from the drop in value of the swap.
How would I work out the resulting weight as % of the portfolio's original value? Is it simply a case of -20/0.98 = -20.4%? A mathematical explanation would be appreciated.