How to find False discovery rate of a ML model? Situation explained below
In your case, you can estimate the FDR by
$$110 / 400 = 27.5\%$$
the approach of sampling some data and estimating the empirical FDR in there is valid, it's the same as estimating the mean of a population using a sample, it makes sense, and the more data you use the more confident you're going to be on your estimate, as long as you're sampling randomly.
400 seems a pretty decent number to estimate FDR, so your estimate is going to be pretty good.