I'll use my combo square or a T square if the distance measured is short enough.

 

Yes but how

 

If I have a lot of blinds to hang, I just make a template out of scrap wood. Start with your measurement on your preferred side, drill you holes in the template and then drill your holes in the wall using the template. Flip the template over and drill the holes on the other side of the window. That way, all holes are the same.
Stiff cardboard will work as well. I have also used an old deck of playing cards in a pinch

 

Your way works as well as any. Nothing to be embarrassed about there. Whats embarrassing is to mis-measure and have to do it twice.

 

measure once....cut twice !!

oh wait....

 

For small distances like that, a combination square is easier, I find, but nothing wrong with a tape measure either.

Rather than putting pencil marks all over the wall, I first put down some green painters tape, and mark the tape. 1-1/2" wide or 2" wide. Don't get the blue, its too dark, hard to read.

 

You're working on a plane, and need an up and down and a left to right, two measurements, so yeah, you've got it. More often than not I will use a folding rule for things like that, but it's only a matter of personal preference, and comes out the same with a tape measure. And I'll often use tape as SPS suggested, especially if I want to step back and take a panoramic view, but if you're locked into a specific width or whatever and have a good eye sometimes you can get by with some little marks that won't show anyway.

 

I don't what made this pop back into my head, but I mentioned having to account for up and down and left to right, or essentially the x and y axis, and you sort of implied being a novice, so I probably should have mentioned the z axis as well. Electricians are required to keep the edges of any of their holes a minimum 1-1/4" from the face of the stud, or, if this is not possible, they are required to place a steel nail plate on the stud to protect the cable. So keep in mind that any time you drill, nail, or screw into a stud the penetration should not exceed 1-1/4", and if you hit a steel plate STOP. I seriously doubt that any of the hardware you're using on this project is any where near this deep, but something to stick in your back pocket.