In custom format. Add more options to show signs for numbers:

.3 ----- 3 decimal places. old way
+.3  ----- 3 decimal places, always show sign for numbers, if positive, show +, if negative, show a short minus sign -
-.3 ----- 3 decimal places, if positive, no sign, if negative, show LONG minus sign
-+.3 -----  3 decimal places, always show sign for numbers, if positive, show +, if negative, show a long minus sign -