No space between overline and letter in Math


when I use this formula P_{overline F}( overline M )={P( overline {M} intersection overline F)} over { P(overline {F})}=2 over 7 there is not enough space between the overline and the letter. Especially the overline in the subscript is not visible.
How can I fix this?

From within a Math object, go to Format → Spacing → under Category select Symbols and change the value for the main spacing (first entry) to something around 5 %.

You can make that value the default for all new Math objects with the second button in the lower part of the dialog.