Hello Joy,
Below is an aerial view of Sandymount and you'll see that Section R isn't really any closer to the pedestrian entrance on Hallhill Road or the vehicular one on Gartocher Terrace.
Sections R & S are together on the same 'block' as you'll see.
Once you reach the correct part of the cemetery, have a good look for any numbers you can find on the bases of any of the stones. Some will have numbers, others won't, but these will be the lair numbers and if you can find a few, you might be able to work out which ways the lairs run in each row.
Alternatively, visit the Commonwealth War Graves Site and look for any Commonwealth War Graves listed in Section R. With a bit of luck, one of them might have a number near'ish to the number of the lair you are looking for and it's much easier looking for one of these stones and make that your starting point.
See:
http://www.cwgc.org/find-war-dead.aspx?cpage=1&sort=name&order=ascThe above link should take you to the first page for Sandymount.
On this CWGC database, rather than call a Section a Section, they are called Compartments, so any you come across in Section R, take a note of the details of a couple you think might be nearby then try and find them first as because they're so recogniseable, they're usually easily found. (The reference can also just be listed as R.9999 or whatever.)
Good Luck!