That's exactly why I said usually. For me, they drop quite often when farming blight spots on planets... I would say 1 engram each 3 or 4 3oC. But the first time I tried it, took about 10 for the first drop. Then 2 more for the second, then 10 again for the third. Thing is, in the long run the average drop rate will be close to 1 engram each 3-5 3oC.