The theoretically perfect frontline is a circle, whose center is placed at your warpgate. This would be the most effective solutions regarding distance and thus the duration to reinforce a base from your warpgate, but it would also neglect all strategic aspects of the different bases. It depends completely on the situation assessment of the leader to find the most suitable and sustainable territory to engage.
The picture above shows another example of a good and clean frontline, where aircrafts and reinforcements can move mostly safe from all your territory to the enemy. You can also see an engagement going on (blue arrow) to remove the last enemy base, which is disturbing your ideal frontline.
On the other hand, this kind of frontline cause also disadvantages, since some bases or frontlines are too far away from your warpgate, what leads to significant delays, if you decide to reinforce those areas.