Could you explain how you got the 18.86?
If I try to recalculate it, I get a different outcome. I've been out of the IC scene for a while, so I could be wrong.
1 uu matter = 1.000.000 eu
Scrap provides a 6x boost for 5.000 eu
UU cost with scrap = 1.000.000 / 6 ~= 166.666,67
Number of Scraps per UU = 1.000.000 / (6 * 5.000) ~= 33,33
So we need 33.33 scrap and 166.666 uu for a piece of uu matter.
To produce the scrap needed:
~33.33 scap = (1.000.000 / (6 * 5.000)) * 45 * 8 = 12000 eu
total EU per UU = (1.000.000 / 6) + 12000 ~= 178.666,67
The optimal ratio = ~178.666,67/12000 = 14,89:1 ~= 15:1
So I'd say you need a recycler for every ~16 eu.
The formula I'd use would be:
R = EU/15,88
or easier to remember
R = EU/16 + 1
Like I said before, I've been out of the IC scene for a while, so I might be missing a crucial fact.