How big a water change you need to bring a level (nitrate, TDS, anything that dilutes) from where it is now down to your target, allowing for what your tap water already contains. More on the why in water changes.
A water change swaps out a fraction of the tank for fresh water, so it dilutes anything the fresh water does not itself contain. That is why your tank can never drop below the nitrate in your tap: you are always adding some back. The fraction to change is (current minus target) over (current minus tap). Keep changes routine rather than heroic; more on why in aquarium water changes, and on the numbers to watch in water parameters.
The fraction is (current minus target) over (current minus tap). This calculator gives both a percentage and litres, allowing for the nitrate your tap already carries.
No. The new water is not pure, so changes alone cannot go below your tap level. For lower, use live plants for nitrate or RO water.
For a large reduction, two or three moderate changes over a few days are gentler than one enormous one, each dechlorinated and temperature-matched.
Dilution: the fraction to change is (current − target) ÷ (current − tap). Since the new water is not pure, your tank can never go below the level in your tap water with water changes alone; to go lower you need plants (for nitrate) or RO water. For big reductions it is gentler on fish to do two or three moderate changes over a few days than one enormous one.