What I do when I've got something like that to do is like Martin said make a template and I can use that to mark the brackets and also the mating piece of steel. I drill 3mm holes at the position I want them and then I've got a 3mm centre punch (old snapped off drill bit ground down and sharpened) that I punch the marks with. That way I can get both sets of holes matching, as long as I clamp it in the right place of course :)