So just to be 100% that I understand how it works.  If I wanted a permanent supply of Inferno grenades (rare, singular, common) it would be +20 and I would have to roll (barring that number changing) 77 or lower?  For a rail gun (near unique, singular, common) it would be -30 and I would have to roll a 27 or lower?  Can I roll them both first then assign them?   ;)