A relatively unknown command modifier is Mid Between Two Points. What it does is prompts you to select two points and then it will find the middle of those two points. You can use this command modifier along with other OSNAPs as well.
For example, say you want to start a line midway between two points. You would start the LINE command and at the prompt type either MTP or M2P (both do the same thing). You will be prompted with:
First point of mid:
If you have running OSNAPs on, you can snap to any point. If you don’t have running OSNAPs on but want to use one, simply type one such as END to select an endpoint. You will then be prompted with this:
Second point of mid:
Select a second point just like you did the first and it will start the line midway between the two points you picked.