adjacent trace capacitance (horizontal)
This is determined by copper thickness, gap, dielectric constant, d and length and thus there is incremental C and L with resulting impedance , Zo for a coplanar microstrip and thus C can be computed from this.  For Zo=50R it is about  3.3pF /" or 1.3pF/cm
So compute Zo from a coplanar stripline calculator (complex) then compute C from Zo as shown here.  This assumes other conductors are >10x further away. If not then you have a mesh of calculations ;) 
If you can't find a coplanar stripline tool, try this formula.


\$ C [pf/cm] = 0.12 * \dfrac{t}{w} + 0.09 * (1+d)*log_{10}(1 + \dfrac{2w}{g} + \dfrac{w}{g}^2)\$


w = width of track [mm]
t = thickness of track [um]
d = dielectric constant ~ 4.2 FR4
g = gap [mm]


hopefully, I got the units correct...you do the math on a 50R pair.

The second term dominates. Using k=3.5 (kapton), w=10 mm, s = 0.5 mm, t = 2 microns, the capacitance is 1.1 pf/cm

Reference https://www.physicsforums.com/threads/capacitance-of-coplaner-adjacent-plates.130876/