With most of your listening appearing on 800mhz then you want to probably make this frequency the primary frequency. It will also result in the shortest antenna. Tuned for 800mhz your reception on 150VHF will only work well if you're inside a strong signal. Forget tuning on 150 for distant stations. If you're close to the transmitter you should be fine though.
Here is how to cut your antenna for 800mhz broke down at wavelength.
1/4Wavelength: Cut down to 3 - 17/32in.
1/2Wavelength: Cut down to 0ft. 7 - 1/32in.
7/8 Wavelength: Cut down to 1ft. 0 - 5/16in.
Full Wavelength: Cut down to 1ft. 2 - 1/16in.
Full wavelength will obviously return best results.
OR tune it to 400mhz which would be an in-the-middle compromise for the full length you want to cover.
1/4 Wave: . 7 - 1/32in.
1/2 WAVE: 1ft. 2 - 1/16in.
7/8 WAVE: 2ft. 0 - 19/32in.
Full WAVE: 2ft. 4 - 3/32in.
If you want the fractions converted to decimal points, I can do that as well. If the whip is removable from the magmount using an allen wrench, I think your measurements start from the very end of that whip. If someone could chime in where the length of antenna truly begins on a magmount.
I'm far from a mathematician but this site has a javascript calculator.