every bit matters, and given that the mast is too far forward on those boats, forward of the mast is non-good!
Anyway, something needs to balance the engine and anchor. Best to get a battery as close in weight to those two, and mount it to balance them out. Note that those mounting locations are as close to the middle of the boat as is practical, which means that any pitching moments are minimized. A really "sneaky" (competitive) owner would weigh his boat 100% empty and without corrector weights, and then figure out how much his optional "stuff" listed in rule 6 needs to weigh to get the boat up to 1270kg and then buy up to that weight before the boat is weighed for measuring. You'll note that the class rules specify a battery MAX weight of 25kg - why in the world would they specify a max weight, surely you'd want the lightest possible right... well, what if you had a light hull and wanted to get it up to weight without putting corrector weights in (which must go forward and aft of the bulkheads and are therefore BAD)? Note that your battery placement is forward of the bulkhead, something a lot of people fight to avoid.
Weight and weight placement matters a lot on a J/24
Similarly (and yes you weigh a lot more than a battery) the helm should only be aft of the traveller during a tack or a gybe. Weight should be as close to the area between the mast and the companionway as possible - ends of boat = baaaad! Downwind you could consider having someone sit forward of the shrouds (basically touching the shrouds and not a bit further forward) to help pull the back of the boat out of the water a bit but maybe your forward battery takes care of that one