CI impact on OPT ALT? Often zero and rarely more than a couple hundred feet. And on Boeing's if OPT ALT has a significant change due to CI it typically impacts MAX ALT. The exact amount depends upon the CI entered, a/c type, and the weight of the a/c.
Restricting yourself to 2000' or 4000' below OPT ALT will typically force you down into the low to mid 30's. In tropical areas I'm typically scratching for all the altitude I can(at least OPT ALT and possibly higher depending upon a/c type) and restricting myself to X thousands of feet below OPT ALT is the exact opposite strategy I'd use.
Eg. OPT ALT 370. The techniques mentioned would restrict you to FL 330 or FL 350. I'd chose FL370, perhaps FL380, and depending upon a/c type perhaps FL390. The difference between FL330 and FL370/380 or 390 can be significant.