After browsing some examples of FS setup, I came up with this variety which seems to work well.
This is using a Futaba T8FG Super, Futaba R6208SB Rx, and a FrSky SBUS to PPM converter.
Turn on your Tx, and load up Mission Planner, plug in the APM to your computer and connect, go to Initial setup, Mandatory Hardware and click on Fail Safe. Turn on Fail Safe, choose what you want it to do, LND, RTL etc, and set FS PWM to 990.
The default value for the Throttle PWM in Radio Calibration is 1100, I tried this and the props spun when ARMED, so I decreased this value using End Point in the Tx to display around 1050 in MP, explanation below.
Go to LNK-END POINT-THR, with Mission Planner open, I raised the END POINT on the low side until the THR Value in MP read 1047, this displayed the END POINT value below.
Now, back out to your Tx front screen and lower the THROTTLE TRIM to +25 (this will be zeroed shortly)
Now you go to LNK-FAILSAFE, scroll to THR-HOLD and change to F/S
You will notice that underneath POS in the R/H column there is a 0%, our next destination, scroll to it so it is highlighted and press and hold RTN until a value of -130% shows up.
When you scroll off of here, go back to the top and highlight FAIL SAFE, double tap, hit RTN and return to the Tx's front screen and return the THR trim to ZERO, and you are done
Leave your Tx on, MP connected to APM and return to the THROTTLE CALIBRATION page, whilst looking at the THR Value, 1047, turn your Tx off and you should see.......
REMEMBER to complete a full RADIO CALIBRATION and go and try it out. I have tested this in my back yard several times and it has worked well.
I hope you find this useful.
Keep it shiny side up.