Derivation of the relationship between clock speed, internal timers and WPM equivalent speed

The following is a table of frequency of occurrence of the alphabet in plain English text taken from a cryptanalysis text, and the length of Morse Code encoding of those characters.

Letter	Frequency (%)	Elements	Contribution
E	0.1300	4	0.520
T	0.0920	6	0.552
N	0.0790	8	0.632
R	0.0760	10	0.760
O	0.0750	14	1.050
A	0.0740	8	0.592
I	0.0740	6	0.444
S	0.0610	8	0.488
D	0.0420	10	0.420
L	0.0360	12	0.432
H	0.0340	10	0.340
C	0.0310	14	0.434
F	0.0280	12	0.336
P	0.0270	14	0.378
U	0.0260	10	0.260
M	0.0250	10	0.250
Y	0.0190	16	0.304
G	0.0160	12	0.192
W	0.0160	12	0.192
V	0.0150	12	0.180
B	0.0100	12	0.120
X	0.0050	14	0.070
Q	0.0030	16	0.048
K	0.0030	12	0.036
J	0.0020	16	0.032
Z	0.0010	14	0.014
Total	1.00000		9.076

Average word duration will be 5 * 9.076 + 4 or 49.38

This represents 60/49.38/10 S (122mS rounded to the mS) per dit interval (Baud) at 10WPM. With that figure, the WPM speed is 2.44 times the dit frequency

I note that an older ARRL handbook suggests for electronic keyers that the WPM speed is 2.4 times the dit frequency, but it does not provide the derivation. This is about 1.6% lower than the above derivation.