Nice work, but I think you're looking at it the wrong way. You said you're in for 305k cash. You cashed out 70% of the appraisal for 308k, so it sounds like you ended up with your entire 305k cash back, another 3k cash "bonus", and then 30% equity in a 440k duplex that generates around $1200/month NOI. On paper, you have to view it like you paid 30% down for this new investment, which is 132k. So your upcoming cash on cash is around 11%, and ROI is around 15% (assuming 5% interest on the mortgage). Those both sound good to me. The cap is 7-8, but frankly, it doesn't mean much without knowing your market.
Now if you want to look at the total ROI of your investment right as you're about to start managing the duplex, it's simple. It's just the 440k appraisal minus what you put in, divided by what you put in. So (440k-305k)/305k=44% ROI over X period of time. Everything here is phenomenal as far as I'm concerned, but nowhere would I say your cash on cash is 512%.
Really, the only question to ask yourself is what you could have done with the extra 30% of 440k. If you had another deal you projected to generate more than 11% cash on cash per anum, then you should've sold this. If not, then you made the right call.
Of course, this is all predicated on the property actually being worth 440k. As other people have mentioned, it's only worth what someone is willing to pay, which could be far less than 440k. Either way, it's still fine to use the 440k number as the basis for your calculations...because it's the best thing you have at this point. Anyway, good luck. I would kill for something that good...and in your shoes, I absolutely would have held the duplex.