My practice has been to deduct approximately 5% of the point value for the problem. So if the problem is 5 pts, I usually award 3 for correct set-up, and 1.5 for accurate calculations and half a point for significant digits/units/etc.

It's a half point for sig figs /units, or any other info that might be needed. So, it's a quarter of a point for each taken individually. Sorry if that was unclear.

Just some perspective. In my first year of graduate school and as a TA, my course instructor required us to take account some students were solving the numerical part of the problem with a slide rule, and most were using a pocket calculator. Intermediate steps using slide rule can lose a lot of precision. Even now, (I suppose, because I have not been teaching for a while), you may have to account for different constants.

For example, my course instructor told us, if your students want to use g = 10 m/s2, that is OK. We are not interested in the numerical answer, but if they know the method. I dislike computer scoring for similar reasons. The computer can sometimes require the student to be "exact", and be unforgiving.

I prefer to take a relaxed attitude towards significant figures. I think you should bring to the student's attention that the number of places the calculator supplies is (usually) to many places. I would bring up this issue with the course instructor and your fellow grad TA's considering you want uniformity in course grading amont all the TA instructors, anyway.