To make your building square 18'x70' the number you need to be square is 72' 3 5/16". Measure from one corner to the other corner/diagonal hope you have a tape long enough. If you cant do it that way then with a transit, that has a plumb bob on it, you need to set it up starting where your first corner hole is going to be, have someone hold the stick or any straight pole where you think your second corner pole is going to be then set the dial at a 90 degree spot w/out moving the optical,once you have set your first position then turn your optical to the right on the 90 degree spot. That will give you a sguare corner. Then you will have to resetup to do the other side. Because it sounds like your trailer is aready there??? But if its not then that number I gave you will work as long as you can measure w/out nothin in the way. Hope this helps but someone else will say that isnt right. Try it, it will work.