There may be some change of the shape in that area of the glass on the front surface due to the distortions that happen from the Twyman Effect. I suspect that there probably won't be much as you're a fair distance away. The only true proof will be to test with either another known good flat or with an autocollimation test.
It sounds like you have a flat that can do about any secondary that you or anybody else would attempt to try for a scope as most of the surface will still be within the specs of the flat. The breakage will affect only the area around it and not the entire flat. I'd have to look at it in person to see what the real effect of the chipout would be. It also could be a clean break that isn't affecting anything on the surface.
A truly lucky find!