How much cement sand required in 100sqm area plaster of thickness 12mm single coat

How to calculate cement sand in 100 sqm area plaster of thickness 12mm single coat ? 

 Let's take propotion is 1:6

Now,

  Total area = 100sqm

Thickness = 12mm

So,

     Total qnty of cement sand                                                                        = 100×.012 = 1.2cum

So 

cement = 1.2/(1+6) = 0.1714cum

Extra cement for voids of sand is taken 20%

So,

0.1714×20/100 = 0.03428

Now Total cement = (0.1714+0.03428)

                                  = 0.2056cum

Now take 20% extra for uneven surface of brick wall

So, 

0.2056×20/100 = 0.0411

Now,

        Total Cement = (0.2056+0.0411)

                                 = 0.2468 cum

                            OR

 0.2468/0.0345 = 7.15 ~ 7.2 Bags

(Here vol. Of 50kg cement bag is 0.0345)

Now sand = 0.2468×6 = 1.48 cum

 FOR DOUBLE COAT

CEMENT = 13.113 bags