When I batch sparge I completely drain the mash tun, measure the collected volume, subtract that from my desired boil volume, divide that by two, and do two batches with that volume of water.
I actually get better efficiency that way than fly sparging.
To answer the question, in addition to the things mentioned above, try not being too gentle with the stirring. The idea is that the liquor drained from each batch is uniform in concentration, so things like lautering slowly and other concerns of fly sparging are irrelevant. Just stir thoroughly, vorlauf, and drain.
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