Disk2VHD image backup with automatic removal of old images when backup drive is full

I've got the following script that I'm running as scheduled task each day.

It makes a VHD image of C: drive (447 GB) and copy it to F: drive (3,63 TB) and removes old vhd images when there is more than 4 of these.

I would like to modify it so it would take current disk space usage from C drive and compare it with current disk usage of F drive. If there will be less space on F than it's needed to backup whole C then the last image would be removed.

It would be good to use fsutil utility:

C:\Users\Marek>fsutil volume diskfree c:
Całkowita liczba bajtów wolnych           : 244490219520
Całkowita liczba bajtów                   : 480127946752
Całkowita liczba dostępnych bajtów wolnych: 244490219520

C:\Users\Marek>fsutil volume diskfree f:
Całkowita liczba bajtów wolnych           : 324223422464
Całkowita liczba bajtów                   : 4000575385600
Całkowita liczba dostępnych bajtów wolnych: 324223422464

REM
REM A simple backup system using disk2vhd
REM
REM version 1.3, by J.E.B., 2011-02-22
REM
REM requires 'disk2vhd.exe' to be in the path
REM

setLocal EnableDelayedExpansion

REM "DRIVES" can be one drive identifier with colon, multiple separated by spaces,
REM or asterisk for all.  
REM "DEST" can be a drive letter or a UNC.

SET DRIVES="C:"
SET DEST="F:"

REM Keep most recent 4 VHD files in DEST, delete the rest

for /f "skip=4 tokens=* delims= " %%a in ('dir/b/o-d %DEST%\*.VHD') do (
del %DEST%\%%a
)

REM Backup to VHD

C:
cd \
DISK2VHD %DRIVES% %DEST%\%COMPUTERNAME%--%date:~-10,3%%date:~-7,2%%date:~-4,4%.VHD

EXIT /B n