A successful spinner-weaver of roto spun open-end cotton yarn must have good and stable operating conditions which include acceptable levels of strength and strength consistency. Not having to defensively react to cotton induced variation allows mill management to constructively concentrate on the finer points of cotton milling. Assured cotton strength performance can be obtained by overkill with the excessive use of high strength cottons in untested mixes or by the more precise method of blending individual HVI strength values.