I saw a cute new kids quilt I want to do for Linus project. Question I have is what size do I cut the half squares to match up to my 8 1/2" focus blocks. The alternate block will be a pinwheel. My thought is to cut the squares 4 7/8" and they will come out 8 1/2 inches. Am I anywhere near right?

Right 4-7/8" is the exact size needed.
However, you might want to cut the squares a bit larger - like 5.25",
make the HST blocks,
and THEN trim them down to 4.5" UNFINISHED.

Yes, it's an extra step, but it gives you a bit of wiggle room, and makes for perfectly sized blocks.

Thanks for the info. Will do amd when I get it done will post a pic. It is adorable.

I just made a pile of pinwheels and what I did was cut them larger as MTS said. Then I layed them out and just squared up the 2 sides that would be sewn into the pinwheel, trimming the smallest amount needed. Sewed the pinwheel, THEN trimed the whole thing to an 8 1/2 inch block. I had better luck this way.