How do I calculate the concentration of the 18.54 mL of the diluted standard sodium hydroxide solution?

Well, you would need to know the strength (molar) of the solution first. So, the Mr of NaOH is 40. 1M would be 40g of NaOH to 1 litre/1000ml/ 1dm^3 of water. 1000ml / 400ml = 2.5. 40/2.5 is 16g in 400ml of the NaOH. Just because you have 18.54ml of NaOH, it does not mean the concentration would be any different as the concentration is just the amount of NaOH molecules to water molecules, so you would have less water AND NaOH, but the same concentration. So therefore, the concentration of the diluted Sodium Hydroxide solution would be 16g/400ml or 16g/0.4dm^3 or 16g/0.4L or 16g/400cm^3. Hope this helps