When determining seeding rate the most important consideration is the expected emergence and establishment. This demands a valid germination determination which will be a reliable indicator of seed emergence.  The lower the germination percentage the more variable field results are. In crops like maize and soybean only seed with 90% or better are preferred and treatment would be recommended in lower germination seed.  Seeding rate is much more critical to yield of maize than soybean because soybean has much ability to respond to an erratic population by branching. In conditions of optimum water, the populations that will yield optimally are greater than in more water limited environments. Finding the right populations are based on specific environments and need some empirical determination. 

Sowing standards are calculated using the following equations:

For fused-area cultures

SR = A * N / 10*EV

Where

SR - sowing rate

A - 1000 seeds weight

N - number of seeds per square meter

EV - Economic value of seed (germination %)

For trenches

  SR = 10000 * A / ID * DP * EV

Where

SR- sowing rate

A - 1000 seeds weight

ID - inter-row distance

DP - distance between plants in the row

EV - Economic value of seed (germination %)

 Recommendation of sorghum and sudangrass is 10 to 40 kg/ha seed. I understand you are testing using broadcast seeding. I would suggest using 10, 20 and 40 kg/ha as experimental treatments. Broadcast over your plots and lightly disk the seed to give a 3 cm cover with soil. This should be done when the normal rains come or irrigation should be practiced. Take data on the treatment reaction in relation to crop performance and issues such as disease and insects this will give a curve where appropriate population for your condition can be pinpointed from the respones curve.