Not sure I follow you regarding setting it to a specific map ? If I'm misunderstanding just reply back
The Grid reference is made up of two letters followed by usually 6 numbers which will give a reference to within 100metres on the ground, usually plenty for map reading. On the Etrex 10 the numbers are in two groups of 5 giving a 10 figure grid reference accurate to within 1metre on the ground which I would suggest you wont be able to visual on a map at a scale of even 1:25000.
But regarding setting it to a map, the Grid reference letters determine a specific 100km square unique to that square. The following numbers can be replicated every 100km hence why the first two letters are critical at least for the GPS as if you only used the numbers for example calling MRT out with your position, without the letters you could be in any one of the 50 or so 100km squares across the whole of the UK
On the legend of your map you will find a small key indicating the letters relevant to your map, and depending on the area it may well have more than one set of two character letters if for example it covers the boundary of two 100kn sections. The letters are also shown on the edges of the map.
Link to one of many maps showing the National grid letters.
https://www.google.co.uk/search?q=ordna ... lwfxdIM%3A