I did this with my Blackbird too. I made one section with two fixed loops and one whoopie sling with 3 inches of adjustability because I wasn't sure that I could get the size just right. The whoopie sling allowed me to fine tune the length to 100".
I think the rule of thumb for buries in Amsteel is 2 fids where a fid is 21 times the diameter. So, applying that to your Dyneema, if the diameter is 2mm, the bury should be 2x21x2mm= 84mm= 3.3 inches. Three fids would be about 5 inches. Rope experts, please correct me if I'm wrong.