I am the other half of the "we" in the OP above.
Some further explanation:
Looking at calcs for lugs on a head, EN-13445 Section 16.6.3 defines equivalent diameter, Deq as follows:
Deq = Ri for spherical sections
and again
Deq = Di×SQRT(1-3((x/Di)^2)) for 2:1 S.E. heads
these numbers...