coolant level 640d

[scubachris]

hi.
my last 7 cars have been e92 330d and 335d and 3 x 640d..
in the 3 x 640d and e92 330d coupe lci the coolant header tank has a "LOW" and a "FULL" mark.
if i topped off any of them to "FULL" mark,they would expell a little amount of coolant (i guess from header tank cap) until they ended up on the "LOW" mark..then they would stay at that level for years.
if i topped off to "FULL" at anytime,it would again expel coolant till dead on the "LOW" mark.
so i guess i should just leave my current 2017 640d on the "LOW" mark.but bieng ocd i would like it half way between "LOW" and "FULL".

can anyone tell me out of curiosity why this is?
from "LOW" to "FULL" is only approx 250ml.

also ,does coolant level drop ever so slightly as ambient temperature drops?.
thanks.

[scubachris]

p.s.the 640d s are f13 s..im in the uk.