Doing layout work on square tubing is easy. You need a decent machinists square and a scribe.
When you mark the location of a hole on one side, set the square with the straightedge perpendicular to the length of the tubing on the first mark, and scribe around to the opposite side. When the line is scribed all the way around, the ends will meet if some attention is expended during the exercise.
With a little care, the holes on opposite sides of the tubing should be within/less than .010" difference. That will probably be as close as just drilling thru the tubing can attain.
It's a good practice to pilot drill holes with a 1/8" split-point drill for holes of 1/2" or larger. When the layout work and the pilot holes are placed accurately, the larger holes are generally (nearly) perfectly located, and cut substantially faster.
Everyone working with metal should have a variety of split-point drills in sizes up to about 1/4". The split-points start cutting immediately upon contact, unlike a conventionally ground drill point.
The web in the center of a conventionally ground twist drill is similar to a cold chisel with a slightly crowned edge. This edge isn't a cutting edge, it just displaces metal as force is applied, before/until the cutting edges contact the metal. The trouble is, the drill continues to require a significant feed force to continue drilling. For larger sized drills, the force required to get the cutting edges into contact with the metal just generates unnecessary heat, then continues to generate more heat until the hole is finished.
It should be obvious that a good cutting lubricant is going to be a major benefit. Application of a small amount, as little as a quick swipe, with a small paint brush (or disposable acid brush) before starting each hole is generally adequate. Laboratory wash bottles with a dip tube and an extended spout are a handy method of applying cutting lubricant for drilling, milling, sawing and turning on a lathe.
Again, cutting lubricant doesn't need to be flooded on, just enough to keep the cutting edges wet, so it's not going to be an expensive practice.. instead, it will reduce cutting time and extend the time of the sharp edges on cutting tools.
For large holes, a good guide for the size of the pilot drill is the width of the chisel edge of the web. Then the pilot hole will allow the large drill to start without a lot of force. The objective is to generate chips, not watch a drill spin around.
As I've mentioned before, creating split-point drills isn't a difficult task. By examining a factory-made example of a split-point drill (a larger one is easier to see), one may notice that there are flats ground at a low angle to the point, on the trailing/back sides of the flutes.
The back side grinds are made first, then the point is ground in the usual manner, and the split points will magically appear.