Seasol use about 6-10ml p/ ltr normally. If spraying will be hard to do I would suggest bumping it up to 15ml per/ ltr.
For urea it is one teaspoon per 5 ltrs or thereabouts.
and re blackberry - I've used brush off mixed with glypho and it's doesn't really matter what the g/l of the glypho is - in my experience it's there as a helper - - that brush off is great stuff
knocked the Blackberry dead - built a fire on top of it, and I returned the areas back to pasture