Just to add spice to the mix, what if the lower were at FL97, assumed to be at FL95 +/- tolerance off 200', and the upper at FL98, equally within tolerance? 100' is enough then...
Then the lower has infringed Controlled Airspace by 200'.
For the UK, I would argue the tolerance is for confirming a reported level when checking Mode C, or for level occupancy using Mode C for validated and verified aircraft, not an accuracy tolerance for assuming someone is clear of Controlled Airspace. If the aircraft is unknown and unvalidated/unverified, I now have to treat it as an infringer and try to acheive 5NM or 5000' separation against it. I can't say 'Oh, he was within +/-200' of the base on Mode C, so I'll just ignore it' unless I want to get a few weeks holiday on suspension .... mmmm, with this weather, there's an idea
