I did the military version of this for my last job pre-retirement, using the VA charts and tables.
Some general thoughts:
1. If you have an injury or illness, tell them. Even if you don't get compensated it's on the record as service connected and you should be able to get treatment from the VA in the future. You may have other health care now (Tricare) but who knows what Congress or DOD will do to that while you're still "working age."
2. Specifically regarding back pain, again I'd just tell them. If the examining physician is doing it right, he or she will be watching for pain. There's a case called DeLuca which requires the VA to consider pain upon multiple repetition of movement when rating. If you're working through the pain there's still pain.
3. Spartana is spot on about concurrent receipt. Above a 50% disability retirees get retirement + VA compensation. Below that the VA amount is subtracted from the DOD amount, but there's different tax treatments and garnishment rules on VA compensation.
4. Calculating amounts: Easiest way: Talk with a veteran's service organization rep, such as the DAV, VFW, or AL. They're volunteers who usually do a great job walking veteran's through the process.
Easy way
http://www.vvaarizona.org/va_comp_calc.phpSlightly harder way:
http://www.benefits.va.gov/COMPENSATION/rates-index.aspHard way. Take highest ratable disability. Multiply by 100. Knew "good" part of you is new figure. Apply next highest rating to new "you." Round up if last digit 5 or higher, round down if lower. Lather, rinse, repeat until you gone from highest to lowest.
The part to remember is that you're not adding them up, (e.g. 50+20 = 70) but applying the disability percentages to the remainder. (e.g. 100x50%= 50( so 0+50=50 "new you") , then 50 x 20 =10, 50 (new you)+10 = 60 (newest you) ratable compensation. I'm not going to talk bilateral factor for now, but if you were to have problems with both legs or arms, for example, there's yet another adjustment.
6. I agree, there's a pretty big jump between 40 and 50 as well as 90 and 100 on the compensation tables.
http://www.benefits.va.gov/COMPENSATION/resources_comp01.asp7. Another long discussion cut way down, if you were in the service 22 years nearly everything is service related, by law. There are exceptions but they don't apply as you exceeded 8 years. And any illness that occurs within 1 year of separation is considered service related, and therefore treatable. But you need documentation.
8. Going back to number 4, I'd highly recommend you talk with one of the veteran's groups expert volunteers. They help screen your record for stuff you may have forgotten or dismissed. Full disclosure: I didn't do that but I lived this process for 2 years and saw thousands of others cases.