As a rule
of thumb, when you are sailing to windward, with the headsail sheeted in hard, you want the angle of the headsail sheet to be at 45 degrees to the horizontal, so use whatever block location gives you closest to 45 degrees. If this is the car on the track (and it should be), then you have the added advantage of being able to move the car to suit (a) different size sails
, and (b) different wind
conditions (ie in stronger winds, move the car aft so open the sail leech, to "spill" air from the top of the sail.
The fixed block may be for "free-sheeting" for reaching.