dura blocks are nice blocks,they're better than the hard rubber ones i started out with.i wouldn't be too concerned with a 1/16th of an inch,for what you're doing the primer will take care with that.when i started out i had the same problem telling the difference,sometimes running my hand over the sanded layers of paint was confusing too i thought it was high or low.if you think somethings low,block it in one direction,if its low the scratches will only be on the high spots around it.a kid i worked with used to run a paper towel over it with his hand flat on top of it.he said it helped.telling the difference between high and low is something that comes with time doing bodywork.have you used a guide coat before?