Creating a 10 ppb Lead Nitrate Solution
Your dilution approach can be heavily improved. 

Rule no. 1: 2+2 $\neq$4 in solution preparation from an analytical chemist's perspective. An extreme example is that you add 50 mL of water+50 mL of ethanol $\neq$ 100 mL solution. If you add a solid NaCl to 1 L of water, the volume may not remain 1 L. It may decrease! 

Analytical dilutions are always prepared in highest quality volumetric glassware (Called class A glassware), which comes in fixed volumes. You might have 2 mL, 5 mL, 10 mL, 25 mL, 50 mL, 1000 mL flasks in the university. Similarly, pipettes comes in fixed volumes. The graded ones (so called Mohr's) are not that great.

With these limitations, you design dilutions, in such a way that volumes are not fractional so a volume of 24.75 mL for dilution is a no-no.

The dilution relation is handy $$\mathrm{C_iV_i=C_fV_f}$$, where i and f indicate initial and final concentrations (C) and volumes (V).

So think about it now in order to prepare 1000 mg/mL you would like to dilute it 1000-fold in steps to make it a 1 ppm solution or 1000 ppb $\ce{Pb(NO3)2}$. From there you would like to dilute it 100-fold to make it 10 ppb.

The first question you would like to think now, is what volumetric glassware is available.