Unless I'm mistaken, switch C center is nuetral. To adjust the center position you would use subtrim and adjust the up and down positions with the endpoint. If your desired center flap position is not achievable with subtrim then you need to add a mix. For something like 0-20-45 you should be able to get that without the mix addded, for 0-15-60 you most likely would need a flap to flap mix.