For reasons of economy, an upper limit of 5 percent by weight, {HEAD ~ preferably even only 4.5 percent} by {TAIL ~ weight}, should not be exceeded 
by
For reasons of economy , an upper limit of 5 percent by weight , { HEAD ~ preferably even only 4.5 percent } by { TAIL ~ weight } , should not be exceeded
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH