The level off arrow will only be very accurate in VS mode if you fly the actual selected or managed speed as the FMS assumes you fly the actual speed and the actual VS. If for example you select 220kt but use a VS-2500 in descent, your actual speed might be around 280kt but the FMS uses 220kt for the predictions and therefore in that case you will reach your target altitude further than the level off arrow. As homebuilt mentioned, A simple calculation using time and VS works much better. Time from the ND or GS. If you fly at 300kt that’s 300/60= 5nm/min. If you have a VS -1500 you will lose 3000 feet every 10 nm.