lowering an upper markup on the staff

[olicha]

In order to show where the separations between music cells are, I wrote:
tic = \markup { \beam #.5 #2 #.5 }
...
r16 c b^"a" a g e^\tic d^"b" f e8
which produces
<http://lilypond.1069038.n5.nabble.com/file/n158718/tic.png> 

I'd like to get the tic lower, at least on the two upper lines of the staff.
The better would be half-stem, if its position could be referenced to the
note itself.

I tried
r16 c b^"a" a g \once \override TextScript.Y-offset = #-1 e^\tic d^"b" f e8

Surprisingly, nothing changes from #-1 to #-4, and suddenly, with #-5, the
tic falls down under the staff.

question one: what is the method to get that tic on the staff?
question two: what is the correct syntax for writing the override in the
definition of the variable (tic = ...)? It would make the tune more
readable.

Thanks



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