Don't take this as an authoritative answer, but your question made me blow the dust off R&E as well as E3:
I can't find artillery cover factors in the main body of the rules either, but there are two sets of cover factors in the quick-reference-sheets: One is marked small-arms; the other doesn't but bears the same factors.
Assuming Empire III can be used as a precedent:
The modifiers for cover are identical for small-arms as well as artillery:
Firers:
Disordered x0.50
Skirmishers:
Super-heavy x0.10, heavy x0.10, medium x0.10, light x0.10, open x0.20
Semi-skirmishers:
Super-heavy x0.10, heavy x0.33, medium x0.33, light x0.33, open x0.50
Formed units:
Super-heavy x0.10, heavy x0.33, medium x0.50, light x0.90, open x1.00; vs flank x2.00, vs square x2.00; vs closed-column x1.50
Artillerists:
x0.33
E3 calculates percent chance per firing figure or "tube".
So French divisional battery (E3 Fire Class I):
To 640 yards: 8 "tubes" @ 8% = 64%
To 400 yds: 8 @ 12% = 96%
To 200 yds: 8 @ 15% = 120%
To 80 yds: 8 @ 25% = 200%
Then apply modifiers. Each 100% post modification is an automatic kill.
paints brigs for the British fleet.
