Maintaining visual during a merge is key, but you should know how to maneuver your aircraft too. Understand the corner speed and turn rate of the Su-27. There is a good video from flankertraining.com that talks about this. It's old and the numbers might be outdated, but I still find the lesson helpful and revisit it from time to time. I tend to use the vertical scan mode when in close range as I find that has the best results (for me anyway).
Link to the video:
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