Unless you are trying to bounce your signal off the moon, just mount your antenna in the best spot you can, as high as you can get it, and it will work fine for for highway/trail comms.
My antenna is mounted above the rear tail-light/rear fender of my jeep, below the roofline, where many "experts" warned me it "will not work". and while it certainly does not work as well as it could, it works just fine. I can hit a repeater with it at 92 miles and regularly get 20+ miles on simplex and have never had issues on the trail where i could not hear someone that others could, or they could not hear me.
My two words of advice are:
1) ignore the 'experts'
B ) - dont over-think it.