Tablespace growth trend

Hi experts,

I have the below details with me.How to calculate the tablespace growth between two dates.

sample data(have data upto 1 year):


INSTANCE_NAME      DATE     TABLESPACE_NAME      MB_ALLOC  MB_FREE     MB_USED   PCT_FREE   PCT_USED    MAX
---------------- --------- -------------------    ---------- ---------- ---------- ---------  --------    ----
ORCL1          28-AUG-14 CUSTOM_TBL               1024       1017.38       6.63      99.35        .65       1024
ORCL3          28-AUG-14 SYSTEM                   1200        277.94     922.06      23.16      76.84       1200
ORCL1          28-AUG-14 USER_IDX                 11000      3159.56    7840.44      28.72      71.28      11000
ORCL2          28-AUG-14 USER_01_TBL              41000      7440.88   33559.13      18.15      81.85      41000

How about

(size2 - size1) / (datetime2 - datetime1)

?