Hmm... Well, your 1988 chevy TBI computer is expecting it to be set to 0... and that's exactly where you should put it. A degree here or there is only making your computer think your nuts. As primitive as these first computers were, the Chevy engineers worked this all out on slide rules and fingers and toes. Setting it + or - wont matter much, as the computer will run what the Chevy boys told it to do way back in the day... including adjusting out your silly attempt to fool it.
Setting it a couple here or there is what is known as an old wife's tale. Any gain you think your getting is a placebo effect. You can however go outside what your old man computer can compensate for.. and while your blasting Quiet Riot on your six by nines, your pistons and heads are making neat dots in the metal. Oh sure, she don't have a reluctor wheel to know for sure what you set it at... but it's going to use it's knock sensor, and AFR readings from the Ox sensor, and it's gonna expect certain numbers to be true.
Set it to Zero.