If your retracts simply swing inward or outward (and don't use bellcranks to convert the motion to backward, like in the Corsair) you can measure the exact amount of throw the retracts need to to go from fully into uplock to fully into downlock. Just measure the total available movement of the unit's actuation arm. Example: one of my units' arm moves 13 mm for a full swing up or down and has an additional 6 mm at each end of this for the locks. Subtract a millimeter or so from each end - you want the retract to go fully into lock at both ends but not to jam against the stop. (13 + 6 + 6 - 2 = 23) Find an output arm for the servo that has holes not over 23 mm apart across the center and not less than, say, 21 mm; enough to cover the swing and get 4 mm into lock at each end. You don't want to be too marginal going into lock. It's annoying when your gears won't stay in downlock and keep collapsing when taxiing. Better yet, get a blank servo output disk and drill your own holes 23 mm apart (or the correct distance for your units) and install your screw-lock pushrod connectors (EZ Connectors) in them.
 
If you have a good multi meter or ammeter that has a range of, say, 0 to 1 or 0 to 2 amps and know how to use it, you can check the actual drain. Get a pair of red and black banana plugs that will plug into your meter and a servo extension; a longer heavy-duty one is more convenient. Cut the positive wire and strip insulation as necessary for the plugs. Put the red banana plug on the side of the cut that leads to the receiver (male) end of the extension and the black one on the side that leads to the female (servo) end. Now connect the banana plugs to the meter and the servo extension between your retract servo and the your receiver retract channel. Power up the transmitter and receiver and try the retract switch to see how much current the servo draws when it is moving. There should be no current flowing when it is not moving, that is when the retracts are fully up OR down. If there is a current draw, mechanically disconnect the retract units from the servo - take the output wheel off the servo but leave the servo connected electrically to the meter and receiver - and if the drain stops you will know that something needs adjustment; a retract is reaching the end of its throw before the servo is reaching the end of its throw, a wheel or gear leg is hanging up somewhere or the pushrods or something in the mechanical linkage is binding somewhere.

Of course a meter with this setup can also be used to check for total system drain by plugging your meter in between the battery and the receiver or to check any other servo for binding linkages .