Relative Scene Launching

I'm trying to build a patch to allow *relative* launching of scenes - i.e. to be able to set Master scenes (these will be hard-mapped to my controller) and then launch scenes relative to them - above them. This is to save me having to hard-map loads of scenes for every song - I just get to the start of the song with one mapping and then everything else is relative to that. I'm not limited by just going 'scene up' every time but I can still e.g. skip back to the 'intro' part.

My idea is to have it look for 'mstr' in the scene name, and then take the base / MASTER scene number from that. I've managed to make a gate that will open when 'mstr' is detected in a scene name, but I'm having trouble making sure it's shut after the next scene is launched.

Total noob here and I'm sure there might be a better + / or more elegant way of doing this, any suggestions welcome!

Hopefully the patch itself will make my objective a little clearer if the above makes no sense ;)

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NB. This is based on ideas in zangpa's excellent Trigger Scene 1.0 patch: http://www.maxforlive.com/library/device.php?id=449