I'm worried I'm doing my dilutions wrong.

Alright, so I work in a lab but right now I'm only doing dilutions on Mn standard.
It's a 10,000 mg/L standard.
So to start I want 100 mg/L so I talk 1 mL of the stock and add that to 100 mLs of water.
When I make a 20 mg/L solution, I should take 20 mLs from the 100 mg/L stock but then do I add 100 mL of water, or 80? This is where I am confused. I want 100 mLs of the solution but I am unsure. :/

from a mole balance...
M1V1 = M2V2

rearranging...
V1 = V2 x (M2/M1)

if you want 100mL of diluted solution
V1 = (100mL) x (100mg/L / 10,000mg/L) = 1mL

so you take 1mL of the standard solution and place it in a 100mL volumetric flask and then dilute to the line

you do NOT take add 99 mL of H2O to 1mL of the stock solution because density isn't constant and therefore volume is not conserved. 99mL H2O + 1mL stock solution ≠ 100mL of solution!

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same thing for the 2nd part.. assuming you want 100mL of 20mg/L solution...
V1 = (100mL) x (20mg/L / 100mg/L) = 20mL

pipette 20mL of the 100mg/L solution into a 100mL volumetric flask and dilute to the line with DI water