You probably already know, but according to the guide, scheelite is found within a tungstate vein (levels 20-50), even though scheelite will the most common ore in the vein.
So now I see why tunneling horizontal is such an awful idea... the veins are only 6-7 blocks tall, so if I'm tunneling at elev 35, right in the middle, I could still miss a vein located at, say, 22-28... or a vein at 42-48. I've never used this "scanner" spoken of, but does it have a range of 8 blocks in all directions? If so, then perhaps tunneling is still valid. But vertical exploration shafts are clearly superior because the only thing we need to do it is wood, which is a 100% infinitely renewable resource.
I wish the GT world-gen could "tilt" the ore deposits for more realism... and spread them out over a larger vertical distance, so horizontal tunneling didn't have this problem of near misses. Have the vein begin up around 50, but spread it diagonally downward to 20, if you know what I mean. No more flat mining where you just find the ore then dig horizontal until it ends.