The usual approach is to measure the CFD's contribution to total portfolio returns.
So if you put on:
a long that cost you 1000 (or was marked-to-market as such at period-end)
a long that made you 1200
a short that made you 800
a short that cost you 800
=> Net P/L = +200
So if you had 10,000 capital at period-start, you'd have made a 2% return. From ...