I've seen several styles of split-level homes. Assuming an entry, then up to one level and down to a second level that is beneath the first, then I put the landing area in with the lower level as the first floor, and take the second level less the entry area as the 2nd floor. If the home is essentially one floor, but split for elevation changes, I show it as one floor, but show stairways and comment. If I have a main living area that is split at one end up and down for bedrooms, again I take the lower section and show it as part of the first floor, showing stairs, etc, then take the upper section as a half-story.
Just my way.
Roger