That of course depends on both the bioassay organism and the chemical being tested. Before scientists begin an experiment, usually they search through published scientific literature for papers that relate to the procedure they have in mind. This is a good way to get an idea about an appropriate range of concentrations. Ideally, you want to test concentrations that cover both of these endpoints plus a range of concentrations in between. Then you will be able to conclude whether your test organism responds in a predictable way to the compound you are testing.
Serial dilutions are one way to set up a broad range of concentrations.
Once you have collected data using an initial set of concentrations, you may find that it would be useful to carry out a follow-up experiment using a more narrow set of concentrations. For example, if none of the seeds sprout at one concentration in your series, and all of them sprout at the next level of dilution, it would make sense to carry out a dilution series between these two concentrations in order to further define the sensitivity of lettuce seeds to your selected compound.