If your language in the mendix developer is set to English, you should be able to enter 0.25. If it is a language where the decimal separator is a comma, you will have to enter 0,25.
Normally you will see the default value of the float in your input field (e.g.: 0.25). There you can see if it is a comma or a dot.
PS, just Entering .25 will never work.