The above method will work, but i highly advise against it, unless you have a fixture made up to do so. A few issues:
Using the press alone is not safe. The spring could pop out and do serious damage to anyone near-by. I have built a fixture to do this, and it basically makes it so the spring cant pop out.
Second, measuring needs to be precise. even a few thousandths of an inch can affect the rate. If you are looking for accurate rating, again, build a fixture using a dial indicator.
Third, it is not simply measure an inch and that is the rate. You need to compress the spring about an inch, then zero the scale, and compress another inch. This rating is much more accurate...
If interested, PM me your email address and I will send some pictures of my "spring rater"