Add sparge water in
order to be able to run off 1/2 of what you want to start with in the kettle
Lauter/Collect Batch 1 So, we want to collect 30.5L of wort from the mash tun in two batches of 15.25L (approx)
During the first batch there will be losses involved, loss due to absorption by the grain and loss to the dead space in the mash tun, this is space in the tun that can't be drained.
The dead space can be calculated by partly filling your empty mash tun then emptying it via the tap. Pour the remainder into a measuring jug and this is your 'dead space'..
In my tun and for the following examples, the dead space is 1 L.
The next loss is grain absorption, this works out at 1L per kg of grain so a 4 kg grain bill will absorb 4L of liquid,
Add the two losses together to give you the amount that can't be retrieved, In this case 5L. Anything volume above this can be drained from the mash tun.
Using a 4kg batch in a mash tun with 1L dead space as an example,
First Batch
To collect 15.25L of wort from the first batch you will need
15.25L + 4L (absorbed) + 1L (dead-space), total 20.25L of liquor.
Concentrating on the mash itself for a moment, most people prefer to mash in a water/grain ratio of around 2.5L water to 1kg of grain. For a 4kg grain bill that's 10L of mash liquor.
In this example you would mash in 10L then once the mash is complete, add the remaining 10.25 L before sparging in order to be able collect the desired 15.25L of wort
The Second Batch
Once the mash tun has been run off for the first time, anything you add to it now will be available for collection because the grains are already saturated and the dead space is already full so it's just a case of adding the volume you want to run off, in this case another 15.25L.
*If the first run off fell short or if too much was collected this can be compensated for in the second batch by adding more or less sparge water during the second batch.
for example if 14L was collected in the first batch, add an extra 1.25L to the second batch to compensate.
To Sum Up using the above figures
Mash using your usual mash liquor ratio eg.
2.5 : 1 Add sparge water in
order to be able to run off 1/2 of what you want to start with in the kettle
Lauter/Collect Batch 1
Add remaining sparge water, this will make up
the difference between what you have just collected what you want to collect in
total,
approximately 1/2 the total volume
Lauter/Collect Batch 2
Boil the wort, which should evaporate down to
leave you with the brew length + the volume lost to the hops and trub
Cool and run off into the fermenter to collect the required brew length leaving the wort lost to the hops and trub behind...
Boil
Volume Batch Sparge Calculator 
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Ale (example of batch sparging)
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Sparging 1