Also, about the coal: have you every played command & conquer? Usually it's a good strategy in early to mid game to start collecting the spice as far as possible from your base as soon as you can (without slowing your building processes too much and while players are still busy building weaponry and the basis for production so that your harvesters can usually defend themselves sufficiently) and keep the stuff close to your base for later in the game when shit hits the fan and you can't have unprotected harvesters wandering around the map far from home, taking too long to supply for the increased funding demands and getting killed due to lack of protection.