Water changes with sump

[Fenton]

This sounds ridiculous I know. New tank with a sump filter. As physics dictates when I syphon water from the tank, the tank water line says the same, but the sump almost empties. Tank is 2x2x2. Filter is 4x1x1.5. is there a method to work out what 25% is for a water change for example of to stop the filter emptying?
I know this is going to be really simple but I'm new to sump filters. Used an fx5 external job before.

Thanks

 

[snake8myelbo]

Turn the pump off then suck out the water.

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[Gill Blue]

After I start draining, I leave the pump running until my sump is about 50% empty. I turn off the pump and drain out 50% of the tank. then I refill the tank until it overflows the sump full again. stop the water and start the pump.

just eyeball it. who cares if it's 25% or 30%.

 

[Fenton]

Fair point their gill blue! And thanks to both replies, the reason I'm waning to know how much has gone out is because I upgraded the tank just over a week ago now, used all the old gravel, live plants, driftwood and water and just don't want to take too much out while its going through a mini cycle.