I would agree that you should start with equal as the rule of thumb. You may need to adjust up or down to your liking. I've found that I tend to use more turbinado compared to dark brown sugar as it tastes milder IMO.
22" WSM & Kettle
Former owner: Jambo, Klose BYC, Med. Spicewine, Pitts & Spitts, XL & (2) Med. BGE's, (2) 18" WSM's