Inadvertent focus check ?

I took the lens and reflector out of my modded Big Head yesterday to get some dust out, and because the reflector has been cut around the bottom its dam near impossible to focus.
I’d get it right then tighten the bezel, only to have it move. Once I finally got it, with the aid of electrical tape around the outside of the reflector, I wasn’t convinced it was properly focused.
It looked right, buuuuut I kept thinking it wasnt. Then a miracle happened, I was moving it under a light in the lounge to see if I got all the dust ( after reassembly ) and noticed it would focus the light in to a very small dot, smack bang in the middle of the dedomed LED.
I thought well if its focusing the lounge light right in the centre it must be 100% focused ? Right ?

Or would the light focus there regardless ?