Is it possible to determine or control where a SCV moves during construction?

Solution 1:

No, you won't be able to control the path specifically.

However, there is some micro that you can use to get a small benefit out of it. You could select the SCV and stop construction (make sure you're not cancelling the building) before the SCV travels to the outside part of the building and then restart construction inside the wall, you'd have to do that a lot to ensure that your SCV stays inside the base.

Solution 2:

No, unfortunately this is not controllable.

In earlier versions the problem was even worse (for players playing against Terran), because SCVs sometimes used to go completely inside a building, making them unclickable. Since catching an SCV at the right time (it being on the edges of the building) was sometimes completely up to luck, a lot of players complained about it (in particular in reference to SCVs building bunkers, where it was sometimes completely up to random numbers if you could defend against a Terran contain as Zerg or not), and it was subsequently patched in Patch 1.2.0

• SCV construction movement has been made more consistent.