entering Grid Reference into garmin etrex

[RockHopper]

Hi I am RockHoppers other half, Pete.
I've been trying to set up our Garmin Etrex to use with Grid References.
For example SJ127533 which is the reference for Eyarth Rocks in North wales
I've set it up in the units screen as follows:
position format = british grid
mapdatum = ord srvy GB
the units = I,m not sure about what to enter the options are statute, nautical or metric
To enter the Grid Reference I went to the edit location screen and entered
SJ 12700
BNG 53300
is this correct ?
sorry if this is basic stuff but all this technology is new to us
Pete