isn't this the same as your other thread?
15mm is a pretty big gap. can you talk more about how your shower screen is supported? for frameless or semi-frameless, often the silicon is also used for strength to keep things in place - at 15mm you won't really have any strong bond there ...
if you're fine with the (lack of) that, if its simply a gap under the glass, how thick is the glass?
if its 10mm glass in theory at least, it is probably possible to 'squeegie' silicon under that, but i don't think its going to look particularly nice...