For a good brief discussion on SWR and misconceptions, see
http://www.cebik.com/a10/ant17.html

As for your assumption about SWR, it isn't as easy as figuring the power
ratios.  It really doesn't take a ton of reflected power to increase your SWR,
and there are a lot of reactive components which, along with your transmission
line, affect SWR.

The formula to figure SWR is:

S = 1+the square root of (r/f) divided by the quantity 1 minus the quantity
square root(r/f) where S is SWR, R is reflected power and F is forward power.
If you actually had 50 Watts reflected power with 100 Watts forward power,
that would result in an swr of 39.9:1.

r=50 f=100 r/f = .5 square root of .5 = .7071.
1+.7071 = 1.7071; 1 minus .7071 = .2929
1.7071/.2929 = 39.9:1.

Steve, K8SP
Lansing, MI