How to know which way to do correction burns, and the most efficient way to do them?

[MET?]

After completing my transfer burns to Duna, I want to correct my planetary transfer so that I intercept the SOI of Duna. I know the most efficient way to do this is as soon as possible. But its hard to tell whether I have over or undershot the planets SOI. I haven't achieved an encounter before the correction burn previously, I always need to correct. How do I figure out which way to burn without waiting a while to eyeball it, and thus hurting my delta-v budget. Alternatively, is there a formula that can tell me where Duna will be at the end of my transfer?

Also: Is it more efficient to burn towards/away from the sun, or just burn prograde/retrograde to curve my trajectory outward or inward?(during correction burns)

Thanks