Cargo space/fuel capacity of the detachment's ships could also factor in to how far out they can go, but mechanically speaking, the idea isn't to form a separate fleet. Just imagine it as the circle representing yours expanding to touch the runners - the larger the circle, the longer it'll take for the main fleet to become available in the battle. Honestly, I wish we could command several fleets or at least two, and dynamically merge/split them, essentially allowing you to create a mobile HQ of sorts with your transports, salvage ships, fuel tankers and supplies, while you break off with most of your combat ships to fight. Could even implement ships/equipment that reduce the CR cost of refitting in space.
But I digress, the extent of this suggestion essentially boils down to an ability extending your strategic engagement range . And yeah, the AI would hopefully take advantage of this ability too, because it's all too easy to outrun enemy fleets if their burn speed matches yours, especially with the current S-burn mechanics. Speaking of which a thread discussing them had a mention of drive field jammers or something along those lines, essentially serving the same purpose. While it'd probably be mechanically simpler, I think it makes more sense to have to put a weaker, but faster portion of your fleet at risk to catch fleeing enemies as opposed to simply tapping an ability button and having the target fleet magically slow down, at the cost of CR/Supplies which is pretty inconsequential if you're in the position to spend them.