Feedback data reply is different bytes of ASCII, can we modify as fixed byte?

Hello. everyone

We have some issuses, here our controller couldn't get Feed back value(-1000 to 1000) because the variable of response byte.
Example

We: ?F<CR>
Roboteq: 

F=-1000<CR>       -> 11 bytes of ASCII

F=1000<CR>       -> 10 bytes of ASCII

F=900<CR>       -> 9 bytes of ASCII

F=50<CR>       -> 8 bytes of ASCII

F=-5<CR>       -> 8 bytes of ASCII

F=0<CR>       -> 7 bytes of ASCII


Probably it should add "data length" inside Or it should be fix the maximun byte

such as
--->Fix byte for 11 bytes of ASCII, inset "0" in space. 
F=-1000<CR>       -> 11 bytes of ASCII

F=01000<CR>       -> 11 bytes of ASCII

F=00900<CR>       -> 11 bytes of ASCII

F=000-5<CR>       -> 11 bytes of ASCII


Could you please recommend us?

thanks
Wasin