Tie a string to the tripod and tie a small object to the other end. Focus on the object with the string pulled snug approximately where you want yourself to be. Another tripod can help here since you have no assistant Ready the camera, pull the string up and touch your face with the small object and you should be pretty darn close to where you want to be. Put it to the side of your eye socket in front of your ear to get the eyes precisely there. A stool with a pillow on top to rest you chin on would also be a good thing to use.
Another thing that may help, along with the string, is to find something you can use to help you locate your head. Stand or sit in a position that allows you to rest the back of your head against a wall, for example. This way your head is the exact same distance after you focus and reposition yourself. It will also help keep your head from moving during the exposure.
An 8x10" of your eye! That's 10x magnification or more, which means your DOF will be insanely shallow - can you even get your whole eye in focus?
The string method is good, but you need a way to ensure your head doesn't move after you let go of the string. Try having a strip of numbers on a wall or something and look at them through a small hole - if your head moves, a different number will be displayed. Adjust number spacing and head/hole/number separation according to the accuracy you need.
Attach a piece of wire to your camera. Bend a rectangle corresponding exactly to the framing at the extension and magnification your want. You'll have to mess with it quite a bit to get it right. It will be like a wire frame viewfinder out in front of the camera that is in the plane of focus, and exactly in focus. Now, the harder part; bend it it fit round your face so your eye is just where the focus is. I can't see how you'll do this without some other person to check focus, but maybe you can fins a way. Once you have all that, you could then work single handed and do the photograph you describe.