If you don't have 900 SD at BB4 level, you really need to look into ways to fix that. BVE your crew, and you should reach 900 with no problem. As for the SD at each tier, it really depends. You pointed out that each nation will have different amounts, which is  true. Also, a +11 crew ( being +11 in the stat that a certain sailor type is ) doesn't affect your SD. The Restore stat of each sailor is most likely going to be different, and restore is the stat that directly effects yours Soft Defense. Boosting, Vetting, and Experting your crew will add to the actual ability of each stat, including restore. You should also make sure that all of your support crew is close to, if not higher, level than your Operator. I also want to point out that BVE-ing your crew will make a world of difference, not only in Soft Defense. You will Overheat longer, repair faster, have better accuracy & reload, and have a better chance of surviving. Even if you don't have real $$ to spend on the game, it is fairly easy to obtain credits and buy sailors that are already BVE.