While you can certainly set up "Have, Needs, Wants" tables, the easiest way to do it is with a decent deco program. MultiDeco, for example, has a Nitrox and Topup program associated with it.
This is one of those subjects where a class, by someone who has done it, really has value. Transfil whips, at a couple of hundred dollars, are the least expensive method and provide the best education. Once you have them, they will handle a myriad of issues. That said, if using the transfil for partial pressure blending, your dealing with pure O2 and there are concerns there. Additionally, make sure your fill pressure operator doesn't bleed off before topping of, something they would do as a matter of normal practice.
If you decide to take a course (a good idea), tell the shop that you want to learn it with a partial pressure emphasis. This will better prepare you for the challenges you will run into as a blender and will serve you better as you move into Advanced Nitrox.
Wanna mess with a stick blender? Tell him you want 80%!!
