Now that the dead space in the mash tun is full and the grain has absorbed all it can then you should just about get out what you put in. So you know what amount of water you need to sparge with. The only proviso is that you should not let the runnings drop below 1010 so you don't risk extracting astringent tannins. You can sparge in one large batch if you have the capacity or split it between 2 or even 3 batches.
Add the sparge water, mix and leave for 10 minutes or so.
Drain and repeat the recirculating of the first runnings.
Add this batch to the kettle.
Repeat again with the required volume to complete the sparge giving enough wort to achieve the required boil volume. Add this to the kettle.
Once you have all the wort in the kettle, mix it and take a sample for a gravity reading. This needs to be cooled to the temperature the hydrometer is calibrated or at least close enough to do an adjustment calculation.