I used the K.I.S.S. principle on this build ( Keep It Simple Stupid) Group 1> Left LLas and ERLLas Group 2> Right LLas and ErLLas Group 3> 2x LLas 2xERLLas (Chainfire)
You can choose to get a ton of AMS ammo instead of the MLas, though I think it is a good backup. You have 0.81 tons free, better strip off a few pieces of armor and make something out of it, you have 2 critslots free.
I don't really get the concept of having ER below cockpit. They're not positioned for sniping, and your weapon groups doesn't even allow to fire them together. It's good to keep it simple, but with 'Mechs we never do stupid I will try this later.
I have my ER below the cockpit because i tend to get my arms blown off in this mech before anything else so figured i would put the best in ST. As far as for how i have them grouped i have them grouped for Right and Left sides.
Ok I will have to explain this in numbers The point of having ER is to reach further, right? Let's look at range, heat and penalty. ER LLas damage fall off: 675m (9 dmg) -> 1351 (0 dmg) non-ER LLas dmg fall off: 450m (9 dmg) -> 901m (0 dmg) So if you wanna do any considerable amount of damage beyond 600m, you gotta use the ER. Why not fire them along with the others? Because of heat penalites or "ghost heat". Only 2 LLas is allowed to be fired without penalty. Numbers again: Simultanous fire: 1 ER LLAS = 8.5 heat 2 ER LLAS = 17 heat 3 ER LLAS = 29.78 heat