Yes, a difference of 0.025em; with a 15px font-size it means a difference of 0,375px. 
But maybe attention for the other super/subscript elements in the page (the not fraction elements), if you use new classes.
For instance: without changing the html the Saml (fist column, 7th row) will stay as it is:
If you use the existing classes, the other super/subscript elements go hand in hand without changing the html over there:
Quite true, Francky. This site has quite a number of pages and I was being cautious since I did not know how the superscripts appear elsewhere and wanted to suggest adding a little margin/padding before it (thus, a separate class might be useful). Color me cautious. 
Thanks both for your thoughts.
I decided that I would create separate classes for the fractions, though to make it easier for me, I named them fractiontiop and fractionbottom rather than sup & sub
As ronpat said, using those may conflict with other census pages on the site.
The styling for 1841 is now completed and 1851 should not be so difficult I hope.
As they say no doubt I shall return in due course but thanks both once again
Antony