If you are using Valbazen Suspension 113mg/ml. your calculations of needing 3.5mls of a teaspoon to get a dose of 400mg Albendazoel active ingredient is correct.
If using cooking measuring spoons that equates to
1. half a teaspoon measuring spoon PLUS one quarter teaspoon measuring spoon.
This is 3.5mls or three quarters of a teaspoon.
I make sure I rinse the measuring spoon off with milk into a small cup to get any clinging to the spoon.
If you need to take your dose out with you don't add milk beforehand because it can curdle it.
The instructions also say not to refrigerate Valbazen/Albendazole.