Here are two useful links for understanding delta kinematics:

Kinematics explained:

http://forums.trossenrobotics.com/tutorials/introduction-129/delta-robot-kinematics-3276/

Online calculator based on above equations:

https://www.marginallyclever.com/other/samples/fk-ik-test.html

I seem to recall that the labels for the dimensions in the calculator are a little misleading, so be sure to double check the drawings in the first link.

Here's where you can enter those parameters into my modified marlin code on github (originally modified from an early version of the firepick-delta firmware). Starting around line 198 in Configurations.h:

#define DELTA_E         50.8//172.0//131.636 // End effector length
#define DELTA_F         158.3//152.4//81.4//190.526 // Base length
#define DELTA_RE        163.5//114.5//270.000 // Carbon rod length
#define DELTA_RF         50.8//77.0//86.000 // Servo horn length

The un-commented values above are the dimensions used on this printer (in mm).