How to figure how many blocks and what size?
 

I am trying to add a border of pinwheel blocks to a panel to make a baby quilt. How does one go about doing that?

First thing I would do is measure the panel to see if it is easily divisible by a number. For instance, if it measures 25", then you could easily divide by 5 and make your pinwheel blocks that size.

Does that help?

Ditto .. and it it isn't easily divisible, then add a border before the pinwheels.
Eg. 23" ... add 1" border all around to get to 25"

Be sure to measure width and length ... as they both may not be divisible by the same number.

That makes sense, but what about my 1/4" seam allowances?

How big is your baby quilt???

Allow for that ... eg. 25-1/2" allows for 25" plus 1/4" plus 1/4"

What are the measurements of the panel that you need help with?? If you post them for us maybe we can work it out for you