You could also just encode precision and/or scale instead. Scale is the number of digits to the right of the decimal point in a number. For example, the number 123.45 has a precision of 5 and a scale of 2.
I don’t know what your number system is but this way you could use a 2-digit major system for the number and then either add a 1-digit image (e.g., a swan) for 2 and this way you’d know where to put the decimal point. Alternately, have different memory palaces… one to store one digit scale, another for 2 digits, etc.
I wouldn’t mix single and double digit systems… either you left pad or you right pad with 0s… so use 12 50 or 01 25 instead. I’d probably go with the left pad as that has absolutely no significance… if you right pad, you could be off if the number was rounded (minor detail).
01 25 with a scale of 1… a “gun” with image for “01” and image for “25” – 012.5
I depends a little bit on your actual situation which system would be most suitable.
A problem I can foresee is that users of 3-digit systems generally use the image for 5 from their 1 digit system to represent 005.
Another solution is hammering a nail into an image: If the decimal point comes after the first digit hammer it in the top, if it comes after the second in the centre and if it comes after the last, in the bottom.
e.g. 123 - TINMAN
1.23 hammer nail into his head
12.3 hammer nail into his chest
123. hammer nail into his foot