A squared plus B squared equals C squared according to Mr Pythagorus.

Not sure how far down you want it to go on the wall.

B

 

Pythagorean theorem

(a^2) * (b^2) = c^2

If both legs are 21" then its

441" + 441" = c^2
882" = c^2
c = 29.7"

 

Also if you make both legs the same say 16" and 16" then you have essentially the diagonal of a square.

Length of brace would be length of leg times the square root of 2.

Your cuts are easier if the legs are the same. 45 degrees each.

 

Got it at 30"! Thanks guys! My geometry really stinks!

 

No problem. Looks like you are off to a great start. Post some progress and finish pics. I'm sure everyone here would like to see it finished!

 

This was just a quickie. Just got divorced and the wife got the house with the big garage and my big workbench. Just needed a place to work on my guns so I threw this together. Also used it as an excuse to buy a new miter saw then realized I have no place to keep it...

Thanks again for the help!