I'm barely halfway - 220 (135K miles) at present. It has slight "fluff" which I usually associate with coolant vaporisation; don't think it's underfuelled as the 270° not shown of the electrode nose look more normal.
If it was coolant there would probably be other symptoms, like loss of coolant, pressurisation of the coolant, water in the oil or V/V, steam in the exhaust etc. Also the plug would be really clean.
Now using '00 Xantia LX HDI, pov spec
My past Citroens :-
'00 Xantia SX HDI, now dead due to accident
'99 Xantia HDI 110 Exclusive, RIP
'97 Xantia TD SX
'96 Xantia TD LX
'96 ZX TD
'89 BX TD
'88 AX GT
'79 CX2400 Pallas (scrapped )
& a couple of Peugeots !