Cruising level tolerances
 

I'm having trouble trying to find the definition of an aircraft being 'at' a cruising level, VFR or IFR.

I seem to vaguely remember that an IFR cruising level has a buffer or tolerance of 50ft either side. That is, if the altimeter is observed to be reading a height anywhere 50ft either side of a cruising level, the aircraft is said to be 'at' that level. Until that 50ft has been exceeded, no clearance or such has been bust. This is on top of the fact that the altimeter need only be calibrated to a tolerance of 75ft so in theory the aircraft could be physically located anywhere 125ft either side of the cruising level.

Are these numbers correct? And also, is there a similar tolerance for VFR levels?