"où C est une constante à déterminer" is the correct translation.
In general yes, when you say ‘to be + past participle’ in English in passive voice, the translation will be ‘à + infinitif’ in French:

• This work is to be done. Ce travail est à faire.
• This car is to be repaired. Cette voiture est à réparer.
• This is to be corrected. C’est à corriger.
• These apples are to be eaten. Ces pommes sont à manger

I cannot think of an exception in French, but I can think of some sentences in French that would not be translated with ‘to be + past participle’:

• We have apples to eat. Nous avons des pommes à manger

The difference is that this example is written using active voice (‘We/Nous‘ is the subject).

The English turn "to be + past infinitive" is not always conveyed by "à + infinitive" (or "à + past infinitive"); instead the verb "devoir" or "falloir" is used. This is so when this turn conveys the notion of obligation.

• Those toys are to be sold soon, the necessity of it has been made clear to you.

1. Ces jouets doivent être vendus prochainement, la nécessité vous en a été expliquée clairement.
2. Il faut que ces jouets soient vendus prochainement,…

In this context "à vendre" is not right. This form introduces a confusion between "sont mis à la vente" and "doivent être vendus".

To express the mere happening of something planned you use the turn "aller + infinitive".

• This type of cultivation is to be ended in the near future.
  Ce type de culture va être abandonné dans le proche avenir.

One is more likely to use "à + infinitive" for negative uses of the form.

• This recommendation is not to be taken lightly.
  Cette recommandation n’est pas à prendre à la légère.

However, the more idiomatic forms of translation use the verbs "devoir" and "falloir".

• This meat is to be eaten very soon, otherwise it won’t be any good.
• Il faut manger cette viande très bientôt, autrement elle ne sera plus bonne.

There is an alternative where the idiomatic turn "c’est" (gallicisme) makes the turn acceptable.

• This vegetable is to be eaten raw.
• C’est un légume à manger cru.
  or, more current      Ce légume se mange cru.