Preload the beam. MAKE SURE THE TOP NUT IS TURNED UP, so as to allow the beam to bend. Find the specs & use a mason string stretched from end to end, measuring the beam deflection as you turn the bottom nut upwards.
Or do as MUMMP suggested to me.
"Turn the bottom nut up til it contacts the beam, then turn it up another 3/8 inch". His method is way faster, so simple a caveman...
Finally snug the top nut down, & crack a beer. Actually, with the money you saved by making a new DS, you & a buddy can crack a couple of cases!
NB the "red" at the bottom of the rod is not rust, merely paint that was applied to the ends of long lengths to identify different rod diameters.