I assume trying to put a number between 0 - 1 results in the tag not being read at all?
You'll need to adjust the REGEX that looks for the tag~
By default, line #257 contains the REGEX responsible for capturing the tag you've mentioned:
InvSizeREGX = /<inv[\s_]+size\s*:\s*(\d+)>/i
This REGEX requires ONE or more digit after ":" and nothing else. Putting "." results in an unexpected character appearing, so to speak, so it no longer matches.
To fix that, you'll have to adjust it to account the possibility of the "." appearing.
For example, if I'm not mistaking, this REGEX:
InvSizeREGX = /<inv[\s_]+size\s*:\s*(\d+\.?\d*)>/i
should, after ":", expect:
ONE or MORE digits
then, ZERO or ONE " . " making it optional
then, ZERO or MORE digits making it optional
So, both
<inv_size: 4> and <inv_size: 4.5126> would be matching REGEX, thus be valid.
Try changing that line of code to what I suggested and see what happens.