You need to get a SWR meter, there are plenty available cheaply that do an adequate job. Also, make sure you have a really good ground before you start; a length of copper braid from a piece of leftover coax, attached to the chassis of the radio and the body of the truck, works very well.
1. Hook the SWR meter up between the radio and the antenna, tune to channel 20 (about the middle of the band), switch the meter to "forward", and key the mic. It doesn't transmit much of a signal without some audio, so I like to whistle a loud, clear tone in the microphone.
2. Adjust the knob on the meter, until the peak reading has the needle lined up with the "reference" or "forward" mark on the scale.
3. Flip the switch on the meter, to "reverse" or "reflected", and whistle again. The peak now should be much lower, and whatever the needle lines up with, is your SWR. 1:1 is ideal, anything under 2:1 is halfway decent. Write it down.
4. Go out to your antenna, and move it around a little bit. Some things to consider: keep the antenna away from other vertical pieces of metal (like the FM antenna), and keep it toward the center of the "ground plane" (your cab roof, in this case) for best SWR and best signal angle. Then go through steps 1-3 again, and see if your SWR got any better. Maybe move it a little more, and see if you can get it any better.
5. Once you've found the best location for your antenna, you can adjust the actual length. There's probably one or two set-screws in the side of the antenna body, just below the vertical whip wire. These hold the whip in place. Loosen them, adjust the length by 1/8", and tighten one screw down a little, to hold it. Then do steps 1-3 again, and see if your SWR is better or worse. If it's worse, you moved the whip the wrong direction; try again the other way. If it's better, then go another 1/8" and see if you can do better yet. Keep adjusting until you find the best you can do. If you need to make the whip shorter, but you ran out of space in the antenna base, feel free to cut a little bit off the bottom of the whip.
6. Once you've got the best position you can find, and the best length, then tighten down the lock screws good and tight.
This will arrange for the antenna to be the best "match" for your radio, so that most or all of the power goes out through the antenna, and little or none of the power is "reflected" back into the radio. The reflected power is not only wasted, it can (in extreme cases) also damage your radio.
Please note that 6W of power into a well-tuned 17" mag-mount, is not as effective as the same 6W going into a well-tuned 4ft whip. Tuning your mag-mount will not make it as good as a larger, better antenna, but it will make it as good as you can get with a little mag-mount.
Let me know if you need more help,
Spott