OK, enough of this "@whoever" bullshit. This isn't Twitter.
When your post comes immediately after someone else's, and you have a quote tool, and you've already said their name at the beginning of your post, adding an "@" is redundant and just plain makes your post look stupid.
There are several fallacies in your calculations. First the GCD is 1.5 seconds, not 1 second. Secondly, you are not giving up an ES, you are giving up a filler, since everything else is on CD it will always be a filler you give up to cast a HM during a fight. You should never be casting mark as the fight begins (except on a few fights where the target is unattackable untill combat begins, Saurfang, Sindragos and Arthas come to mind). Thus the calculation isn't relevant to losing anything else than a steady shot, and last I checked, SS would not hit for an average of 7k at the ilvl of gear that you should have at 5.3k gs (which really doesn't say anything to be honest). Also, you will never give up more damage because the GCD is higher, you can still only fire off 1 shot per gcd, and a sI already sid, you don't lose an ES, you lose a SS.