Can someone help me with this serial dilution?

The stock solution of DNA that I have is 500 ug/mL (micrograms per milliliter) I want that first in nanograms per microliter instead (ng/uL) I assume it’s 5,000ng/uL, right?

Now, from THAT stock solution, I want to eventually make a 100uL solution of 75ng/uL DNA. I need to make a dilution factor but I’m blanking out. It was easier when I had to make 5ng/uL. All I had to do was pipette 10uL from the stock and mix with 90uL of the buffer I’m using and then do the same thing to that again to get it down to 5ng/uL. (So two 1:10 dilutions) But since this is 75ng/uL now, I’m just having trouble applying what I know to this.

I’m aware there are several ways to do this. Such as just making a 5ng/uL solution and pipetting enough to make it 75ng/uL. But I would preferably need a shorter way to reduce pipetting error.