It`s hard to make a decision sometimes, all "common" forenames !!!
Plus, I`ve got several who just seem to have disappeared !
I DON'T KNOW WHAT HELPS AND WHAT DOESN'T . . . 
ANDREW JOHNSTON + SARAH MITCHELL
Marriage: 8th March 1793, Canongate, Midlothian, Scotland
BIRTHS: 
Parents: Andrew Johnston + Sarah Mitchell
Child: Elspith Johnston b: 08/06/1793, St Cuthbert's
Parents: Andrew Johnston + Sarah Mitchell
Child: Janet Johnston b: 23/01/1796, St Cuthbert's
Not sure if the 2 younger children are related, but, it would seem strange to marry in 1793 and not have a child until 1798, it's worth a follow up just in case, and whether also there may have been a second marriage . . .  
Andrew Johnstone + Sarah Mitchell
Child: James Johnstone b: 14/10/1798, Greenlaw 
1841 CENSUS 
Name:          James Johnston
Age:                   45
Estimated Birth:    abt 1796
Where born:       Berwickshire, Scotland
Civil parish:       Greenlaw
County:             Berwickshire
Address:          Westerraw Part of Town Of Greenlaw
Occupation:       Weaver
James Johnston     45
Agnes Johnston     45   B: Berwickshire, Scotland
1851 CENSUS 
Name:          James Johnston
Age:                         51
Estimated Birth:    abt 1800
Relationship:       Brother
Where born:       Greenlaw, Berwickshire
Civil Parish:       Melrose
Town:          Galashiels
County:           Roxburghshire
Address:          Island St
Occupation:       Hand Loom Weaver, (Woolen)
Alison Johnston     49 Head: B: Greenlaw, Berwickshire, (Woolen Yarn Windler)
James Johnston     51 B: Greenlaw, Berwickshire
Christian Johnston    9 B: Galashiels, Roxburghshire (Niece)