Yes, but ....
Your miter gauge must be square to the fence. Take a framing square and test it at 90 degrees. Rest the miter head so it's exactly at 90 degrees, even if it means coming off the built in stop just the slightest bit. Then lock it down tight. Make a test cut on a scrap and check it with your square. If it's good, you all set to make your other cuts.
Your problem is going to be that you won't have 18" in front of the blade to start your cut, UNLESS you pull the miter gauge partially out of it's slot. You may loose some accuracy by doing this. A sled would be a better way but alas ..... no sled.
No easy answers as far as I know to this...... except make a simple sled with a longer runner or 2 runners.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
Last edited by woodnthings; 01-24-2016 at 11:01 PM.