that's a beam style torque wrench. when the bolt or nut you're tightening stops spinning and begines to torque the needle that's free floating over the numbers will begin to deflect. when it deflects to the point it's over the torque reading you want stop turning the bolt. as for being 10 lbs off, that's not normal. there may be an adjustment on the guage to allow you to zero the needle but there might not be. if not, just go 10 lbs further then the suggested torque settings. like Silver said, valve cover bolts are rated for inch pounds. make sure that wrench is reading inch pounds before using it. or just do it like changing an oil filter. finger tight then another 1/2 turn. if you notice oil still leaking, go another 1/2-3/4 turns but don't go too far.
while you have the cover off, hit it with a degreaser inside and out and really clean it up good. check for any cracks, especially along and near the ears where the bolts tighten down on the cover. be sure to clean out the two vent ports on top really good so you don't end up with oil in your air filter later on.
RIP: '88 YJ 2.5L Ax-5 NP231
Posi-Loked. Herculined. Optima yellow top. 1" Shackle, 2" BDS. Cragar 397's Aussie front.
92 YJ 4.0L Ax-15 231
5" springs, 1" shackle 31's or 35's depending on my mood