Basicaly you want to strike a shard off the core that is the width that you want your flints to be, it may be 1-3inches long then it can be broken into pieces the length you want if you take 5-6 flints os a similar size and lay them together with the shap edges along the one side and the flat down you will get the idea of what your core flake should look like. you probably have scads of mini flint flakes around when working your points, you can also strike off a single piece/flake that is the size you need or close and work it to fit with pressure flaking leaving one sharp edge, often these will come of nearly usable with two or three good edges.I imagine some of your flakes you strike off to make small points could be shaped to make a gunflint.Work it cold and not heat treated for gunflints they will last longer.