It's widely known that a 2-second differential rule of thumb is the best way to run the 800. In others words, in order to run a 1:54, go out in a 57 and come home with a 59. Doesn't work for 100% of people, but it does for most.
This depends on the level of ability and age of the runner. The slower the runner the more a differential.
54.5 then 58.5 will get you to 1:53 in a few weeks.