Looking at the Google developer docs on the FAB , there is no single answer to the gap between the FAB, but he claims that
The floating button should be set to 16dp min from the edge on the mobile device and 24dp min on the tablet / desktop.
Knowing this, you can touch the buttons accordingly using the 16dp / 24dp rule.
On a side note .
When I worked with the Google Play Editorial Team to prepare my app for a feature, they indicated that the FAB should be limited to one on screen. If you need more than one, perhaps create an expandable menu. Something like that

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