Dear R B,
After synthesis (coating with silica of MNPs) and washing the beads, we resuspend them in 1:1 (volume) with water. After decanting the beads, we measure (more, less) what is the volume occupied by the beads and add that amount of ddH2O to them to resupend. This is our storing stock. Then, dependent on the application, we use different dilutions of the beads (from the storing stock), the dilution is (or at least should be) indicated in each protocol. Nevertheless, since the synthesis might work slightly different for each user, I would recommend to benchmark the beads. For this, the easiest approach (this is what we do), is to take a known amount of cells (i.e. 0.5 milion HEK293 cells) and do TNA extraction while using different amount of beads. Afterwards, check the concentration (or make a gel) to compare the results.
Hope this helps,
Tomek