A far better approach to the process of getting the number into the controller is to implement a string editor function. Each key is entered into the buffer as it is entered. Do not try to keep a real time version of the number till you hit an enter key. A 4x4 key pad might be better. If you are stuck with the 4x3 then use the * key for a backspace to permit user editing of the input value. A long press of the * key could correspond to an <enter> key. The string approach lends itself to interactive display of the
entry on an LCD screen if you have that.

Only after the entry is complete would you try to convert the number to floating point representation.

Programming is always simpler if you decouple the functions from each other like I suggest.
Michael Karas