And we will not know how we shall serve Hashem until our arrival there. (10:26) If a miracle is performed for a person the person is obligated to bring a Korban Todah - a sacrifice of thanksgiving. Moshe is telling Pharaoh that as of now we have no idea how many miracles Hashem will perform for us. Until we actually leave Mitzrayim and reach Har Sinai we will not know how many sacrifices we will be obligated to bring. (Ksav Sofer)
And Moshe said (to Pharaoh) ‘So said Hashem, ‘At midnight I shall go out in the midst of Mitzrayim. (11:4) Rashi says Moshe said at approximately midnight which means close to it either before it or after it, and he did not say at midnight lest Pharaoh’s astrologers err (as to when is exactly midnight) and say Moshe is a liar. But Hashem who knows his times and moments exactly said at midnight. (Rashi)