I'm sorry. I see that I wasn't clear at all in my response. I didn't mean that the pressure was correct at your altitude. Just that it was correct at your location (at sea level). That's why I provided the conversion factors. Pressure decreases be about 1 inch of mercury (1.0"Hg) for each 1,000 feet that you increase in altitude. It loses goes down by about 0.1"Hg per 100 feet and 0.01"Hg for each 10 feet. If you are near Reno/stead airport (FAA identifier 4SD) your elevation is about 4,214 feet. If you are near Reno/Tahoe International (FAA identifier RNO) then your elevation is about 5,050 feet. Let's say you're near Reno/stead. Your pressure from weather.com is 30.00"Hg. The pressure you would key into your GPSr would be 25.79 (reduced 4.21) for your altitude of 4,214. If you are near Reno/Tahoe Int., you would input 24.95 (reduced 5.05) for your altitude of 5,050 feet. While it's not exact, it gets you into the ballpark.
Again, I'm sorry if I confused. It's usually better to just use the "calibrate by GPS" selection. Just select "no" for the questions of "do you know the correct elevation" or "Do you know the correct pressure." Another way is to simply pick up a topographic map of your area, locate your position and key that in as the "correct altitude."