I find marking 1, 2, 3 is kind of useless unless it is a complicated encounter that requires extremely fast target switching. I have macros set up on my CHB2 (R2, X, O, Tri, Squ) to mark targets 1, 2, 3, 4 if that is ever the case. However, I use this macro to mark targets during combat.
/mark off <t>
/action auto-attack <t>
/wait 1
/mark cross <t>
/party {{PLEASE ASSIST ON <t> }} || Marked with "X" ||
This allows me to change which order I want mobs killed in on the fly. I hit shield lob, and punch this macro right after I shield lob a mob, marking it for the first mob to die. Then, as I go through aggro combos, I see which target needs to die next based on AEs/dps/hp/effects/aggro levels/etc and then I just use the d-pad to target the mob I want, and hit this macro again.
A DPS would have to be near brain dead on only looking at a sliver of their screen to miss the giant X and the /party message, so this has made grouping very very easy. I just tell everyone "Don't attack ANYTHING until you see the mark" which gives me time to build aggro if I need it on groups of 4-5 mobs, etc.
The problem with linking shield lob to your macro is that its going to eat a GCD timer, meaning you can't flash in the interim, you are 1 ability behind on your RoH combo, etc. I know SL builds enmity, but if you are watching enmity meters to begin with you shouldn't need the boost upon the assist call. Just map the above macro somewhere easy to reach and you should be set up for a smooth experience.
If you miss your shield lob macro, I'd use this:
/micon "shield lob"
/action cover <tt>
/wait 3
/action provoke <t>
/action shield lob <t>
This is the "Mob Loose" macro I made. As soon as a mob gets loose, target it and hit this macro, it will Cover your target's victim (target's target=tt; if its not on CD), then wait for you GCD to cool off if you had just hit an ability, then hit it with a double smack of provoke and shield lob, instantly taking aggro back from the stray mob. I like to follow this up with flash immediately to solidify the aggro of the loose mob, as well as snap back on the mobs I just ignored to save my stupid WHM who is overhealing ><