Another hint if you're doing a lot of tubs... I made a template out of one of those big paint stirring sticks that they give you at Home Depot to mix the 5 gallon buckets of paint. I just measured the holes evenly, drilled, then use that when I go to melt the holes in my tubs. Perfect holes ever time, and they all look the the same.
BTW, I did try drilling the first couple of times, but my Sterilite tubs did crack. It wasn't too bad, just small 1/2" cracks by the hole, but it bugged me. Maybe my drill bit wasn't sharp enough or perhaps I pushed too hard. Either way, I like the smooth and easy holes that the soldering iron makes.