Well it is actually 80mv (4.23V) and yes that actually coresponds to about 100mA, but unless you have your N900 in standby with almoust nothing running (WiFi, BT etc) then you rarely get the last drops from the battery anyway, that's why i say you won't feel/get the last runtime.

Again only way is to charge your battery's to 4.23V manually or hack the charging system to allow 4.23V, but a word of caution here go over 4.25V (only 20mv here so very easy to do) then your battery will be damaged, do this few times and you might aswell go and buy a new battery.

EDIT: battery meter is not accurate unless you do all those steps to calibrate (i haven't tried it), and that's why i made my battery meter that is in my battery thread, i got tired of looking at those wrong % readings and such