RunningHeader variable: prevent display of a lower level paratag

For navigation purposes I like to have the main heading and the first subheading on a page display at the top of a page. 

My paragraph tags are defined as follows:

H1 Heading 1 is used for the chapter title

H2 Heading 2 is used for the main heading

H3 Heading 3 is used for the subheading

I use Running H/F variables to pull the text from the main heading and the subheading into the headers. 

Running H/F 1 is defined as: <$paratext[H2 Heading 2]>

Running H/F 2 is defined as <$paratext[H3 Heading 3]>

So far, all pretty standard. The issue arises when there's a new H2 Heading 2 on a page but there's no following H3 Heading 3. The header updates to use the new H2 Heading 2, but it's still using the information from the H3 Heading 3 that goes with the previous H2 Heading 2, and that's no good.

How do I get it to not do that? Or more precisely, if a new H2 Heading 2 is showing on a page that doesn't have a new H3 Heading 3 below it, how do I get the variable to be blank instead of showing whatever the most recent H3 Heading 3 text was? I can't be the only person this has been an issue for, so I'm sure a way around this has been found. I just haven't managed to turn it up yet.

Any pointers?