Changing the angles for better performance is allowed, as long as the plan view doesn’t change. The easiest way to do that is using GemCad or Gem Cut Studio, but read https://usfacetersguild.org/gemstone-design-conversion-using-the-tangent-ratio-method/ for an explanation and methods. Bob’s Rock Shop used to have an online calculator, but the site seems to be unavailable now.
Keep in mind that competition stones are not evaluated for brightness, so it won’t make a difference in your score.