Many variations of this are possible. Best to decide which is the most fuel critical scenario. generally it is the Deoressurised return PNR. The engine(s ) out is not so critical fuel wise because a drift down descent will be the norm.

CC's method is the way to go but it can be tidied up a little by avoiding te SGR out and SGR back bit.

Having arrived at the start point note Fuel on board. Calculate the fuel to return to departure in the non-normal condition plus 10%(Vrbl Res) and deduct this amount plus the Fixed Reserve.

The remainder amount represents the fuel from Start point to PNR and back to Start point including 10% (Vrbl Res)= 110% of fuel burn. Say you have 25300 kg ,pullout your trusty prayer wheel aka Jepp CR something. Place 110 on the Inner against 25300 on the Outer and read against 100 on the Inner = 23000kg

Suppose Normal TAS 480 F/Flow 4400kg/hr

TW40kt

And Return TAS 340 F/Flow 4000kg/hr

Then GS Out = 520k @ F/F 4400kg/h

And GS Back = 300K @ F/F 4000kg/h

Calculate(Prayer Wheel Or Calculator)

[GS Out/GS Back * RETURN F/F] + Normal F/F

[520 /300 * 4000] + 4400

6933 + 4400 = 11333kg

This is the fuel needed to fly OUT for ONE Hour normally and RETURN non normal

Then It gets easy..Prayer Wheel.

Place 11333 on the Inner opposite 23000 on the Outer hold (don't shift) it there and READ ON THE INNER opposite the following:-

60 pointer = Time Out = 122min

GS Out(520 = Distance Out = 1055nm

F/F Out(4400) = Burn Out = 8950 kg

Prove it! By setting Gs Back 300k on the Inner opposite Distance out 1055nm on the Outer.

Then again:-

60 pointer = Time Back = 210 min

GS Back 300k Set vs dist 1055 Done

F/F Back (4000)= Burn Back =14050kg

Proof is that Burn Out plus Burn Back (To Starting point) add up to fuel available for this

Fuel Burn Out 8950kg

Plus Burn Back 14050kg

Fuel Avail 23000kg

Can be used for any number of engine out or non normal configurations. Very theoretical!