Typically their head should be within the range you mentioned. Too far ahead is asking for a tangle, and too far behind you will walk away with one arm longer than the other. Lol
I always always pony from both sides, as it gives me an opportunity to build my weak side, but also makes the ponied horse even. You will notice the pony horse will want to be on one side, or he'll want to keep you on a certain eye. By establishing both sides you will make him confident to both eyes.
If they are constantly trying to get too far ahead, you must bonk the lead rope until they respond and back off. Repetition fixes that one.
If they lag behind, I'd suggest going back into a round pen or an arena to work on the lightness of the horse being ponied. Have the ponied horse in the middle, and have your horse walk in small circles, having the lead rope taught like you are asking it to hurry up his feet. Continue to do this until you feel a give, then give the lead rope and allow your horse a little bigger of a circle. If he lags behind again, close the cicle, pressure on the lead rope until he hurries his feet, then release. Once he understands that a tight rope means walk faster or even trot, you're good to go. I wouldn't hit the trails until the horse I am ponying is light and with me, or else you can get into trouble.
Another way is have someone ride behind you, and if your horse lags, put pressure on the lead rope and have your friend shoo him forwards. He'll learn that way, and it won't be that he will only respond if a horse is behind him, he will learn to understand.
Typically I wouldn't pony a green horse with a green horse. Ponying is meant to give the horse being ponied a stable, confident leader and often green horses just can't give what you need. It can teach them naughty habits,& it can make them more aggitated than needed. Remember that green horses out on the trail are supposed to have fun, pleasant, easy rides until they're confident and willing. It doesn't matter the age, so long as the riding horse is responsive, calm, & obedient.
With your mare she just has to find her place of peace. So make it extremely clear where she needs to be. Get after her when she wants to move ahead, and if you have the room to make a circle if she lags behind, get after her that way. The only place where she gets a piece of mind is at your stirrup. :)
Try also not to constantly hold her back. Have the rope loose unless she surged forward, and then bonk her rope. Having a nagging pressure on her head will allow her to push into the pressure and continue on her merry way. :)
Hope that helps! Good for you to get out there and do that! Ponying is a wonderful tool.
In riding, a horse's energy is like a river- guided by the banks but not stopped by them.