According to the following report on level busts by Eurocontrol it would appear that the 300ft level bust gate is not that strictly defined. Page 2 refers.
But 300ft is what our company uses.
http://citeseerx.ist.psu.edu/viewdoc...=rep1&type=pdf
And it would appear that ATC in the UK at least regard you having reached a FL when they get 3 successive mode C readouts within 200ft of that cleared level. MATS 10.3.1 (CAP 493). So I guess given that our modern altimeters won't let us set 50ft increments then the level bust of 300ft is the next one up from 200ft.