With grapes a refractometer is way better as you can check the brix with just one grape therefore knowing when they need to be picked. Like any wine you need to know what each variation should be. In otherwords a Cab sauv should have around 12-14 abv but you wouldnt want a Pinot Grigio to be anywhere near there. As you know picking to early will result in low sugar levels and very high acidity.