600' mainline?, wow, how big is the property? 1.25 is not overkill, however how much flow are you designing for? You may be able to do a "looped" main, that is you can have a mainline that basically forms a circle around your property. If that mainline is 600', you can figure your pressure losses at half of that, or 300' in this case. By doing so, you may be able to use 1" pipe instead of 1.25".
