Probe offsets, travel limits and boundaries for probing must be set accordingly (values depend also on end switch positions)!

Move your bed to measure distance between nozzle and front/back limit of your bed, move carrier on X axis to measure distance between nozzle and limit of your bed. Add bed width/depth for max pos. Measure distance from center of probe to nozzle for x/y probe offset. Using the probe linked by CarlosCastro you get:

//Probe offsets (line# 590-593)

define X_PROBE_OFFSET_FROM_EXTRUDER -22 // X offset: -left +right [of the nozzle]

define Y_PROBE_OFFSET_FROM_EXTRUDER -45 // Y offset: -front +behind [the nozzle]

define Z_PROBE_OFFSET_FROM_EXTRUDER 0 // Z offset: -below +above [the nozzle]

On my Anet A8 i got following travel limits (travel limits are not the same as build volume!):

// Travel limits after homing (units are in mm) (line# 730-736)

define X_MAX_POS 235

define X_MIN_POS -35

define Y_MAX_POS 222

define Y_MIN_POS -2

define Z_MAX_POS 240

define Z_MIN_POS 0

Using clamps to hold my glass bed in place i need 25mm on the front side of the heated aluminum and the distance from nozzle to probe on the back side.

// Set the boundaries for probing (where the probe can reach) (line# 823-827)

define LEFT_PROBE_BED_POSITION 20

#define RIGHT_PROBE_BED_POSITION 200
#define BACK_PROBE_BED_POSITION 175
#define FRONT_PROBE_BED_POSITION 25

Setting up configuration.h accordingly all probing positions can be reached. Your mileage may vary.