Bring the bike in the house and line up the saddle by sighting to a windowsill or fireplace mantle.

Once I get it set the way I like, using a razor blade, I scribe a thin mark on the seatpost and it's adjustable top section. I used to use a magic marker, but that rubs off, and isn't accurate anyway. If I'm still experimenting, I cut a wedge of masking tape and use the pointed end as a temporary marker on the seatpost.

Road Fan -- a digital level? That sounds like a good tool to have.