Can anyone tell me what components go into calculating this statistic?  I use it all the time when selecting my pitchers.  But when looking at the IP vs HR ... it doesn't always add up tro me.

I believe its based on how well (or poorly) each pitcher did versus the league average for the season.  A "100" rating would be average, with a higher rating being better and a  lower being worse.  Needless to say, a HR+ rating of "120" for the 1999 season is not anywhere near the same as a "120" rating in say...1909. 

I use that statistic only when I am deciding between two pitchers and the HR/9# is very close. 

a 120+ Hr/9 means that pitcher allowed 20% fewer homers that season than his league's average. Likewise, a rating of 80 HR/9+ means he gave up 20% more homers than the league that given year. It's best used in conjunction with the HR/9 and HR/9#s. Viewed independently, you can end up with homer allowing machines like Denny McLain or Catfish Hunter.

HR = Homeruns allowed
IP = IP
LHR = League Home Runs
LIP = League IP

HR/9+ = (LHR / LIP) / (HR / IP) *100

THANKS ... Is it the best stat to use when evaluating which pitcher gives up the most HR's ?

no, HR/9# is. Technically.

Sim HR/100AB		HR/9
		0.05	0.10	0.20	0.30	0.40	0.50	0.60	0.70	0.80	0.90	1.00	1.10	1.20	1.30
HR/9+	50	0.26	0.51	0.98	1.40	1.79	2.15	2.48	2.78	3.06	3.33	3.58	3.81	4.03	4.23
	60	0.26	0.51	0.96	1.37	1.73	2.07	2.37	2.65	2.90	3.14	3.36	3.56	3.76	3.94
	72	0.26	0.50	0.94	1.33	1.67	1.98	2.25	2.50	2.73	2.94	3.13	3.31	3.47	3.63
	86	0.26	0.50	0.92	1.28	1.60	1.88	2.13	2.35	2.55	2.73	2.89	3.05	3.18	3.31
	104	0.26	0.49	0.89	1.23	1.52	1.77	1.99	2.19	2.36	2.51	2.65	2.78	2.89	3.00
	125	0.25	0.48	0.86	1.18	1.44	1.66	1.85	2.02	2.16	2.29	2.41	2.51	2.61	2.69
	150	0.25	0.47	0.83	1.12	1.35	1.54	1.71	1.85	1.97	2.08	2.17	2.25	2.33	2.40
	180	0.25	0.46	0.79	1.05	1.26	1.42	1.56	1.68	1.78	1.86	1.94	2.00	2.06	2.12
	216	0.24	0.44	0.75	0.98	1.16	1.30	1.41	1.51	1.59	1.66	1.71	1.77	1.81	1.85
	259	0.24	0.43	0.71	0.91	1.06	1.18	1.27	1.34	1.41	1.46	1.50	1.54	1.58	1.61
	311	0.23	0.41	0.67	0.84	0.96	1.06	1.13	1.19	1.24	1.28	1.31	1.34	1.36	1.39
	373	0.23	0.39	0.62	0.76	0.87	0.94	1.00	1.04	1.08	1.11	1.13	1.15	1.17	1.19
	448	0.22	0.37	0.57	0.69	0.77	0.83	0.87	0.91	0.93	0.95	0.97	0.99	1.00	1.01
	538	0.21	0.35	0.52	0.62	0.68	0.73	0.76	0.78	0.80	0.82	0.83	0.84	0.84	0.85
	645	0.20	0.33	0.47	0.55	0.60	0.63	0.65	0.67	0.68	0.69	0.70	0.70	0.71	0.71
	775	0.19	0.30	0.42	0.48	0.52	0.54	0.56	0.57	0.58	0.58	0.59	0.59	0.59	0.59
	930	0.18	0.28	0.38	0.42	0.45	0.47	0.48	0.48	0.49	0.49	0.49	0.49	0.49	0.49
	1116	0.17	0.25	0.33	0.37	0.38	0.40	0.40	0.40	0.41	0.41	0.41	0.40	0.40	0.40
	1340	0.16	0.23	0.29	0.32	0.33	0.33	0.34	0.34	0.34	0.33	0.33	0.33	0.33	0.32
	1609	0.15	0.21	0.25	0.27	0.28	0.28	0.28	0.28	0.28	0.27	0.27	0.27	0.26	0.26
	1931	0.15	0.18	0.22	0.23	0.23	0.23	0.23	0.23	0.23	0.22	0.22	0.21	0.21	0.20
	2318	0.15	0.16	0.19	0.19	0.19	0.19	0.19	0.19	0.18	0.18	0.17	0.17	0.16	0.16
	2783	0.15	0.15	0.16	0.16	0.16	0.16	0.15	0.15	0.15	0.15	0.15	0.15	0.15	0.15

Any chance you could interpret that table? I'm not sure what the left-hand column is telling me.

The left axis lists HR/9+, the Top lists HR/9... cross reference and you get HR/100AB or how many homeruns a pitcher will typically give up in 100AB.  For example, you can expect a pither with a HR/9 of .5 and a HR/9+ of 216 to give up 1.3 sim HR per 100AB in OLs.

Many Thanks!