Actually it's Lowell's idea (or one of his associates).

I would suggest just looking at the most powerful flares (x) to see if there is any significance before using the less powerful M flares (etc.).

Canie's is right. The column (12th) with the "X" heading is the one to use (with the date on the far left).

# Date 10.7cm Number Hemis. Regions Field Flux C M X S 1 2 3
#---------------------------------------------------------------------------
2001 01 01 171 119 1190 2 -999 B4.9 7 0 0 4 0 0 0

Extracting all X flares events for 2001, we have:

An X1 flare on
2001 03 29 262 315 3940 0 -999 C3.8 8 7 1 22 2 0 0
X3 on
2001 04 02 228 223 2780 1 -999 C6.5 2 4 3 12 3 0 0
X1 on
2001 04 03 223 228 2600 3 -999 M2.0 3 2 1 8 1 1 0
X1
2001 04 06 192 136 1790 0 -999 C3.3 3 0 1 6 0 0 0
X1
2001 04 10 170 170 930 0 -999 B4.7 5 0 1 4 0 0 1
X1's
2001 04 12 149 159 1030 0 -999 B8.1 2 1 1 2 1 0 0
2001 04 15 134 100 730 0 -999 B5.5 7 0 1 3 0 1 0
2001 06 23 206 228 1590 2 -999 C1.5 7 2 1 24 4 0 0
2001 08 25 199 132 1300 1 -999 C7.2 17 2 1 0 0 0 0
2001 09 24 279 315 3160 2 -999 C1.8 8 0 1 0 0 0 0
x2
2001 10 19 248 219 2110 1 -999 C2.2 8 6 2 23 5 2 0
x1
2001 10 22 233 207 1770 1 -999 C1.3 6 2 1 11 1 2 0
2001 10 25 239 225 1830 2 -999 C1.4 15 0 1 17 0 1 0
2001 11 04 227 186 1820 1 -999 C1.7 5 0 1 12 1 0 1
2001 11 25 170 102 850 1 -999 B9.6 16 0 1 15 0 0 0
2001 12 11 221 154 1430 0 -999 C2.8 4 1 1 7 0 1 0
2001 12 13 220 212 1440 3 -999 C2.1 4 2 1 12 1 0 1
2001 12 28 263 263 1830 2 -999 C2.6 3 3 1 13 2 0 0

To strip this data just read in the 12th column and if it is > 0 then read the date on the left. These are the start of the X flare 5 day windows.

Just taking an eyeball guess real quickly...maybe the number of M7's per year approximately equates to the number of X-class flares per year....hmmmmm.

.02