Some where in the net I saw some huge mathmatic formula for calculating the volume of water acutally changed after a period of time thru smaller waterchanges.
I really wish I had written it down or saved it.
When doing a daily water change or acutally when doing any water change you should always start by having the tank full. If you dont replace evaporation and draw out water from the tank and then top the tank off with fresh saltwater there is the chance for a slowly increase in SG
from adding more saltwater than was removed.
If your like me I dont measure the water removed or added. I just drain into a 5 gal
bucket till its about 2-3" from the top. then I fill the tank back up with water from another 5 gal
bucket that is about as full as the one I drained into.