How to calculate parasitic capacitance in a trace?
Your equation:

\$ C = \frac{e_oe_rLW}{d}  \$ 

Is the parallel plate capacitance equation and assumes that the plate size L*W is large enough and the gap size d is small enough that most of the e-field is captured between the plates.  With your system, most of the capacitance will be in the field lines around the wires so the result will not be accurate.

Usually this would be simulated with software but as a first approximation you should use the capacitance of a two wire line.  This will be some what inaccurate because teh conductors are a significant size wrt to the spacing.  But it will be better.

\$ C = \frac{2 \pi e_r e_o L}{2 ln(2h/b)} \$  

where h = 1/2 the distance between wires and b = the "radius" of the wires.

In your case the wires will not be circular (but they are not square either).  but this is a good first approx..

Some open source software, that is kind of hard to use if you don't have the right tools is found by a search of "fast field solvers"