The label for your product (active ingredient glyphosate) calls for 2% strength for hard to control perennial weeds. Amount of spray per acre will depend on how much you need for the size of the weed growth and other factors. Typical amounts used range from 10-40 gallons of water per acre. Here the link to an abstract for yellow starthistle control that may prove helpful.
http://tncweeds.ucdavis.edu/esadocs/Centsols.html
It states "Glyphosate controls C. solstitialis at 1 lb aactive ingredient/acre (1.1 kg a.e./ha) (DiTomaso et al. 1999b). Good coverage, clean water, and actively growing C. solstitialis plants are all essential for adequate control. Unlike the growth regulator herbicides, glyphosate is non-selective and controls most plants, including grasses. A 1% solution of glyphosate also provides effective control and is used at this concentration for spot treatment of small patches. Glyphosate is a very effective method of controlling C. solstitialis plants in the bolting, spiny, and early flowering stages at 1 to 2 lb a.e./acre (1.1-2.2 kg a.e./ha). However, it is important to use caution when desirable perennial grasses are present. In late season treatments, except with glyphosate and ester formulations, a surfactant should be added to the herbicide formulation."

Here is a link that might be useful: Remuda label

jayk, thanks for the information. I did came across the thread but it was a little vague as to how many gal per acre, I found that it is very important to spray at the right time. I have been mowing to keep good control, but thought I would try a weed killer, such as Remuda to try and get to the root system, in our area we have a very long dry season, but starthistle is very hard to kill off because of it's deep root system, hoping that Remuda will do the job, plus mowing, thanks again.

Dan