Hi! I suggest you try the inputMask column in your XLSForm. You can find info about inputMaks in this help topic. It looks like a value of 9.9 in your inputmask would do the trick.
The input mask controls user input as values are entered in the input box. Incorrect values are shown in red, but once you have a value that matches the mask it will switch to black.
Another approach is to use a regular expression in the constraint column.
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