you may want to have about 1/4 inch nub where it hits the top flange in the middle for those out of square flanges, if i remember i'll take a pic of mine at work tommorrow.
on 800 lb columns (or any size for that matter) where the flanges arent square with the web you need to measure from the center of the flange or you could be way off! 

If I can ever get the boys who borrowed them to return them to me, I will post pictures of the version of the same basic device I use.  Much like yours only designed to lay flat on the face of the beam web, work off the center of the flange and is both delineated to mark with either with a soapstone or use a centerpunch to locate bolt holes at the standard locations at a distance relative to the centerline of the top flange.  The centerpunch used to mark points is nothing special rather it is of the standard hex variety about 7/16 in diameter and is located at its point in a drill bushing.  There are multiple of these drill bushings at the typical hole centers on the side of the "Beam square". Each guides the center puch to it's location and allows for accurate marking.  Should some other dimension be required from the top flange to the center of the first bolt hole shims are used under the upper contact point at the center of the top flange to adjust theis dimension.  A fixture was made at the same time the beam square was fabricated to be able to quickly check the square for alibration in the event it was dropped or otherwise bent.  Here small lengths of drill rod are inserted into the bolt hole center point punch location bushings to check alignment with the fixture.  Bolt hole locations are easily found and laid out with the relative ease you would expect from custom tooling.  It is now very easy to lay out bolt patterns on opposing beam ends using one end of the beam for reference end.  Shop drawings were subsequently modified after this tool was developed to be dimensioned like a cnc drawing  with all important locations being referenced from one end. (zero point)  This eliminates a lot of the problem of tolerance stack up when measuring from cut ends of beams.  I know a picture is worth more than all this description, but my buddy just got a large structural job and borrowed all three of my beam squares and the check fixture.  I fixing someone else's CJP's that did not pass UT and from the looks of things I will be here another month owing to the tedious grinding needed to get the somwtimes three inch long slag inclusions out while working inside a completely finish painted and plastered house.  41 more points to go till I am all done!  Air Arc or plasma are out of the question.  So no need for structural layout equip.  I would encourage anyone to build something like this for their use as it saves a lot of time and prevents a lot of small errors.  Have not had to ream a single hole on the 970 tons of iron I personally fabed and erected this spring/summer.  That alone is enough to make anyone feel good about the time spent to build this stuff.  Let alone how much smoother it goes on the jobsite.  Saves a lot of crane time too when you don't have fix stuff during the erection phase.  Lets see some more of the special tooling you guys make for helping your work go smoother and faster.

heres one of my smaller ones, i also have 1 i use on floor beams and 1 for heavy columns!