how to convert dbfs to db spl?

I played a sound with web audio api and used a gainNode to change the volume,When I changed the gainNode's value, the Decibel meter showed a number not expected.

context.createGain();
var source=context.createOscillator();
source.frequency.value=1000;
source.start();
source.connect(gainNode);
gainNode.connect(context.destination);

gainNode.gain.value=Math.pow(10, dbfs/20));

when dbfs=0,the Decibel meter showed 90 dbspl;
when dbfs=-10,the Decibel meter showed 84 dbspl;
when dbfs=-20,the Decibel meter showed 78 dbspl;
when dbfs=-30,the Decibel meter showed 72 dbspl;
when dbfs=-40,the Decibel meter showed 62 dbspl;
when dbfs=-50,the Decibel meter showed 52 dbspl;

the actual volume was not changed as expected, was there any mistake in generating the gainNode's value? I want to play the sound at 80 db spl or other specific numbers,what can be the dbfs's value and how to make the gainNode's value ?