Optimization is one of the biggest problems in any engineering field. It takes a lot of time and effort, and just taking 3 values will not be sufficient for a well-researched scientific evaluation of the problem. Ideally, the more the values that you take, the better it is. But I recognize that time/computational/storage issues makes this complicated.
What I suggest is that you search for tendencies. If upon changing the angle/entrance length by a small value, you get a tendency towards an optimum answer, you can try a few more values in that direction. Start from the extremes, and come towards the mean.
As user @1318980 suggested, using Matlab is a good way to do this. There are many python codes available online, and you can search through mechanical forums to find one that suits your needs. Another good way to do this is to use special and dedicated optimization software (like Optimus) to perform these simulations (it's easy to understand with simple GUI input for the variables).
Once you get a tendency, you can always go ahead and make further changes to the design before you finalize one.
Thanks and regards,