20 years makes a huge difference in virtually every situation. How much can vary. I would research CRDP and CRSC. CRDP can increase compensation in excess of $3k/month in some scenarios. 20 years also provides important protections for your retirement. There have been examples of those separated instead of retired at 19 yrs, 11 mns.
How to handle that profile can be a big determination how things go too. If you can do the 7 basic tasks they can write it as a P2, which won't auto trigger the MEB. As a 11B, the commander can attempt a reclass with a P2, but generally they don't, and its not automatic. If they do the P3 but you can do the 7 basic tasks that will cause a MAR2 process instead. If commander likes you its fairly easy for the MAR2 to say you can wait out the next 1.5 years. Its beyond common for 11Bs to reach this point at the 16-19 year marks. Its almost standard practice to hide them in a safe job if its a simple no PT profile. Training room, S3, brigade, division, they're like elephant graveyards.
If you can't do the 7 basic tasks, ensure they profile all the conditions, not just the knee or something. Profile is a huge factor in what becomes unfit and hence the DoD rating. There are things you can do to delay the MEB process once started to help reach the 20 years.
Ratings guesses are guesses, I recommend militarydisabilitymadeeasy.com for a easy guide to understanding ratings. You provided almost no information, so any guess is gonna be really bad. My guess is with 18 years as 11B you have a few other things to talk about for VA claim as well. Its important to claim everything with the VA, not just the problems bad enough that you went to sick call for. Even a 0% rating from the VA can be important down the road. Rough guess is 30% DoD as long as a shoulder or knees are unfit along with the back and maybe a 70% overall for VA for what you listed.