What I do with mine is to get the latitude and longitude from my Navman F20 and then I go to this website http://www.gps-data-team.info/map/ and put in the street details and convert it over to decimal degrees and then put that into my Navman F20 and it is spot on every time.
I have also just put in my lat and long details as well at times and it also seems to be pretty accurate for me, but converting it to decimal deg gets it right every time.
When you get to the last entry 20.58 I think this is where it goes wrong for the Navman because you and only enter 20 in the 2nd last block and 5 in the last. You cant actually put 58 in the last block, this is why I use the decimal deg.
However if the last 2 blocks were something like 26.2 this would be ok to use the normal input with lat and long.