If you wish to assess individual capabilities, use an individual assignment. Group projects are fundamentally and unavoidably terrible at doing so. Use group assignments, instead, to encourage collaboration - this is, you may notice, completely incompatible with your suggested marking system, which actively encourages antagonistic behaviours towards other group members.
As far as actual methods of assessing group projects goes, I'd suggest just... not. If for some reason you actually must have a number next to it in some system, grade on completion. If not, just don't write any numbers down. Give verbal feedback to the group, of course, and I suppose you could give them some sort of numerical or letter grade if you really wanted, but I'm not convinced that there's any particular benefit to doing so, and I certainly wouldn't write that number down anywhere (at least not anywhere that matters).