Nil, we are talking through each other on semantics only.
The problem is that you cannot have an equation as you first presented it.
SD cannot equal SD + something else.
So therefore, there is an SD1 and an SD2 for lack of current symbol names.
So you original statement must read:
and if we take the value you provided as SD2: 29.4, we can now solve for SD1.
Trajpar is simply a value progressing linearly from zero to one.
Trajpar at zero, the output value for SD1 will be 29.4
Trajpar at one, the output value for SD1 will be 29.4
Which is exactly what it should be for a circular sweep feature.
Now we also do not know how you are using the formula, other than the fact that it must be a sweep.
If we can get past this set, we can move forward.
Your required shape reminds me of this discussion:
If you patiently follow all the links, I think you can learn a lot about advanced sweep section relations using trajpar parameter.