Any luck OP? You don't have to measure from underneath. Just measure the distance from the center of one handle to the center of the other handle on top (assuming you have a handle for cold and a handle for hot). I think the majority of kitchen sinks are 8", under the assumption that your faucet is old enough to need replacing. Do you have a spray nozzle off to the side? If so, get a new faucet that comes with one. If not, then don't get one that comes with a spray nozzle. The number of designer sinks seems to be increasing rapidly over the past decade or so, with all sorts of options - but if your sink is old enough to need a new faucet, odds are, 8".
And, egads - we put in a bathroom vanity recently. It didn't have the standard 6" (or whatever it is" holes for a bathroom sink. Holy crap - to get separate handles to each side with a center faucet, in the bronze that matched everything else, was an extra $100 over the cost of a standard sized bathroom faucet of the same color.