Ricko,
I expect something in the range of 1 - 20 pF, but it could be more. It's hard to say without seeing the parts. If you have an RLC bridge that works at low RF, it might give you a more exact value.
If you find the value needed s on the lower end, you can make up a variable cap by twisting two insulated wires together and soldering the ends across the cap. In the beginning you can untwist and retwist them to get close, and then later on cut bits off for the fine adjustment. BTW, these were called "gimmicks" and you can sometimes see them where low value fine adjustments are needed.
This only compensates for the parallel capacitance. Depending on how well you want this to balance you may have to go further. By the way, what are the loads on the inductors. Do they add much capacitance?