Roll center height/position of arms

[Aussie_Haynes_MX5]

Can anyone shed some light on what a good roll center height would be for a Haynes? I've worked out the dimensions for where the top and bottom ball joints sit in relation to the wheel and tyre combination i'm going to be using, but from here i will need to know the RC height to start drawing up the geometry. Looks like my ride height will be about 120mm or so.

On the second part of the question/topic, do any of the arms have to be horizontal when the car is sitting at it's designate ride height, or is a matter of as long as the arms are the correct length and the ball joints are in the correct position, it doesn't matter.

Reason for the question, when using the NC MX5 uprights, the distance between the upper and lower ball joints is only about 161mm. This means that when using a 205/50R15 wheel/tyre combination, if the bottom arm was to be horizontal at rest, it would need to sit at 230mm from the ground.

Any thoughts, or if someone can steer me in a good direction, i'd appreciate it. There's quite a bit of calculation and maths that i've had to call on which i haven't used since i left school.