Try starting out with the ridgeline at 100 inches. I have the same hammock and it is fine at 100".
I think the bigger you are the harder it is to find the sweet spot. Sometimes it just takes tightening or loosening the suspension an inch or 2 and it will make all the difference in the comfort. And to make it more complicated each hammock is different. I like a lot of sag in my ENO, 30 degrees in my Blackbird I had to tighten my ridgeline a little, and I sleep best in a Switchback with it pulled tight.
Even a half inch difference on the RL can make all the difference in the world.... Keep tinkering.... Also, try changing which way you lay in your hammock...i.e. which side your head and feet are on.
Custom Bridge Hammocks
Mackinac Bridge Hammocks
And there's nobody here that'll laugh at you.
We'll be thinking that we aren't doing it right after all these nights
It does take a bit more tinkering to sleep in the clouds like GODS....
Keep workin at it. Seems to be getting ckloser for you to be comfie
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Wife(nurse): It's your shoulder blade, dummy.
The hammock forums legend (according to Knotty, I believe) is that a nominal ridgeline is 83% of hammock length. That's 10" for every foot of hammock. So, 100" would be correct for a 10' hammock like the Blackbird. If your hammock is really only 9' long, then you would theoretically start with a 90" ridgeline and play with it from there. With that in mind, your current ridgeline may actually have too little sag for you to really get diagonal! Short people can get away with a much tighter hammock because it's easier for them to get diagonal.
The factory suspension on a doublenest pulls the edges of the hammock tighter than the center. Since the edges on any gathered end hammock are already pulled tigher because they're spread out, the additional edge pull from the suspension sometimes makes them really uncomfortable for me. If you change your whipping on the end to a warbonnet whip, it will relax the edges of the hammock slightly and make it easier to get diagonal which should be flatter. Headchange4u did a great tutorial on how to do that whipping as part of his quest to add a ridgeline to his blackbird.