Sorry for the short version lol.

In all seriousness though. There are several things to consider with making a plane fly knife edge.

The seamaster was not designed with this maneuver in mind. It also was not designed to do an inverted flat spin but several of us got ours to do it. A couple with disastrous results. If you do try inverted flat spin make sure you have plenty of dammit room.

Anyway. You will need to get as much throw as possible. Here are the other factors though. You have to have very good power to weight ratio. If your plane is heavy and or marginal on power it won't work, you could have 90* rudder throw and it wouldn't matter. You have to pull your plane almost more than this plane will fly through it.

Make sure your servos can handle the increased torque needed to push the control surface. The farther you adjust for max throw the more torque you have to have.

I don't know what you have for power or how much your plane weighs so this is the best I can do for you now.

You may need to stiffen up the vertical stab. When your rudder is hard over and you get any flex (which the seamaster is known for) then you loose some of the aero force you need in the right direction.

Have fun and happy flying:
Dave