Okay, so to recap: the maximum effective magnification your telescope can deliver will be the diameter of your objective lens or primary mirror multiplied by 2 (if you use millimetres) or 50 (if you use inches).
You can magnify beyond this but the quality of the image will drastically tail off as you crank up the magnification. A really good (read: expensive) scope may allow a few times more magnification than the effective limit but, as a rule, most scopes won't. That's why a small supermarket telescope offering 400x magnification will be one to avoid - it won't deliver anywhere near the stated level of magnification, and it shows that the makers and suppliers don't have a clue (or a care) about how telescopes work!
Next up is how to calculate the magnification a particular eyepiece will deliver.
You'll notice that eyepieces have a grading system in mm (26mm, 21mm, 7mm etc) when all you want to know is how much magnification they'll give you! That's because the magnification a 14mm eyepiece will give you in a short-tube scope will be far less than in a scope with a longer focal length. For example:
A telescope with a focal length of 500mm (remember, that's the distance between the objective lens or mirror and the eyepiece) will give you 35.7x magnification:
500mm / 14mm = 35.7
A telescope with a focal length of 1200mm using the same 14mm eyepiece will give you 85.7x magnification:
1200mm / 14mm = 85.7
So it would be impossible to put a magnification rating on an eyepiece unless you were only going to use it with scopes of the same focal length.
What Can You Expect to See With Your Scope?
Now for Paul's own patent pending rule of thumb for calculating what faint fuzzies you can see:
If you're in a town or city, double the apparent magnitude of your faint fuzzy or halve the limiting magnitude of your scope as a minimum guide. If you have a 5” mak then your limiting magnitude for a faint object is about 6.5. So even without the problem of low surface brightness the veil is out of reach. To do it the other way the Veil is the equivalent of at least magnitude 14 in a light polluted sky, making it a difficult if not impossible feat for an 8” (remember that low surface brightness) and perhaps just possible with our 16” scope and its limiting magnitude of 15.5.
Outside of urban areas then we are much closer to being able to see the telescopes actual limiting magnitude, but even so, to avoid disappointment count on losing a couple of Mag from your limiting value when looking at faint objects, unless you have perfect seeing, an immaculate optics and the eyes of Tycho Brahe himself.