30% seems high. We take 10% off of the T/O c of g and replace the default 7.5 with that. This usually results in a value of about 20% give or take. (This on the 300ER). Never did it on the other variants that I flew.
What you're doing is telling the computer that your c of g is further aft than the default, therefore allowing a higher cruise level than would have been calculated with the default c of g value.
The default is very conservative. The buffet protection required for that value would limit your climb. This is just a way of communicating a more acurate cruise c of g to the FMC.
I suspect this is just ass covering by Boeing. or by their legal advisors. otherwise, why wouldn't the normal entering of the T/O c of g on the perf. page adjust the default automatically for you? With the default in there, the chance of an upset is pretty remote as you'll be cruising at a lower FL.