How to make sure the pex line is sloped so I can drain it

I'm running a cold-water pex line in a basement crawlspace that gets extremely cold (i.e. freezing) in the winter.

I'm going to use a drainable ball-valve inside the house so I can drain it in the winter but how can I run the pex line to make sure it's sloped? The run is about 25 feet long.

I think I have two options:

1) Run it through floor joists (requires a hole in every joist since my line is running perpendicular to them)

2) Run it under the joists and use clamps and wood shims under each joist to achieve a slope.

I'm leaning towards #1 but to make sure I have a good slope I think I'll need to measure my holes to make sure the line is actually sloping. For example, I thought I would measure a certain distance from the joist to the floor above to make sure my holes are actually sloping down.

Any other suggestions?

(NOTE: This question is sort-of a follow-up to this: Do I need to angle/pitch pex tubing for draining in the winter?)

Thanks!