Concentration
In short Concentration states how much substance is dissolved in a given volume, usually in milligrams per millilitre. It is the amount weighed out divided by the volume of solvent added.
Less solvent means a higher concentration. For small withdrawals a lower concentration is more convenient, because larger volumes can be measured more accurately.
The arithmetic itself is a division: amount divided by volume. Ten milligrams in two millilitres gives five milligrams per millilitre. Because a milligram has a thousand micrograms and a millilitre a thousand microlitres, that is also five micrograms per microlitre: the same number in a different unit.
Two assumptions are built into this. First, that everything dissolves; second, that the powder itself adds hardly any volume. With large amounts in little solvent the second no longer holds exactly, and the final volume is then greater than the volume added. If you need precision, measure rather than rely on the arithmetic.
The third assumption is the most delicate: that the nominal amount equals the quantity of peptide. Because of the counterion and residual moisture, it is an upper limit. How large the gap is for a given lot is explained under peptide content, and the practical steps under reconstitution.