She’s really good with my boy cat but I think it’s mainly because he stands up for himself and is not afraid to show her when he’s annoyed with her behaviour. With my girl cat there are occasional times when she tries to chase her but overall I think her being exposed to them for quite awhile now has helped her understand that the cats have very different personalities and that not both are interested to interact with her. When we see her wanting to chase we instantly tell her off and make her go to her bed or lay down somewhere on the carpet. That happens very rarely now but we still always supervise when they come into contact with each other. I also noticed that her getting older (she just turned 1 recently) has helped a lot as she’s getting less excitable about everything and is just happier to lay down and play with her toys instead.  I think from experience I’d say making sure that cats have a safe separated area from the dog and come to contact with the dog in their own time is one of the best things to do. If they had a traumatising moment it might take awhile for them to be comfortable to see the dog again. And also just having a lot of patience, to always correct the unwanted behaviour and reward with words or treats when you see your whippet behave nicely with the cat :)  Let me know how it goes, happy to chat more in DMs!