I think four three three is getting a little confused as well. FL257 is considered as FL260. Three hundred feet is the maximum allowable discrepancy for maintaining a level which is why the mode C has to be out of four hundred feet and moving in the right direction to consider a level vacated useing mode C. Also if you are in a holding situation, your Mode C readout may well be garbled with others in the stack and be unuseable for separation purposes, therefore we rely solely on pilot reports. If you are asked to report vacated then when you do, make sure you have really left the level. Once you report vacated I can perfectly legally assign that level to the next plane in the stack. If you are still there it could mean paperwork for all of us at the least!!!!!!!!!
(edited for stupidity)