My father-in-law used to do it on his Chev by seeing how long a plack line his tyres can leave on the road... In a Volvo I just tune it like I would a carburettor. In fact, that's more or less what it is. And a Volt meter will load your Lambda sensor too much, can't use that. So listen to the idle speed and fine the rich corner. Then drop it back a bit if you want, but not to the lean corner. You might notice a nice performance, I did with my wife's '88 B200E (came from the Philipines, no Lambda). I did not bother dropping it back to the leaner side, and my wife gets over 10km/L (whatever MPG that is).

Have fun...