I'd rev it until it wont advance anymore(vac adv disconnected) & lock it down at 38. You would need a tach hooked up to know the rpm at that point where it wont adv any higher. Is this a stock dist w stock springs? 38 total will get you pretty close to ideal & if there's no pinging as you run it up thru the gears you're Ok though if it still has the original springs, the light one & the heavy one with the elongated loop on one end you would very likely gain by changing it(still keeping the total at 38). Also note what your initial is at idle before/after(any)changes.