From Prof. Y. Lupardi (http://www.xs4all.nl/~atarist/geo/gps_rd.htm):

Setting RijksDriehoek (RD) coordinates:

The manual of the GPS 12 is very vague about how to set and use the
'User-defined' feature. For the GPS12 (as I have) you do the following:
Goto the MAIN MENU and the last entry there is SETUP MENU. Enter this one
and a list appears. The second entry in the list is NAVIGATION.

After an [enter] you are in the navigation setup page where you find:
POSITION FRMT and MAP DATUM and 'Units'. Set the units to 'metric'.
By default the POSITION FRMT is hddd°mm.mmm'. Move the cursor to this and
enter. Then you toggle downwards until you arrive at 'User Grid'. Do an
[enter] and you find your first page with numbers to fill in.
Longitude origin: E 005°23.258'
Scale: 0.9999079
False E: 155000.0
False N: -5316592.0
Do the 'save'.

Next to do is the Map datum. In the Nav Setup menu goto the MAP DATUM and
toggle upwards until 'User' appears. Do an [enter] and you can fill in the
USER DATUM
dx: 593
dy: 26
dz: 478
da: 740
df: 0.1003748

Important: before you do all this set the 'metric' in the menu 'Navigation'.