I remembered that Naruto Forums has a calculation regarding Pica lifting and carrying the Palace to another location. However, I do not see anything regarding the actual Flower Hill creation itself, so perhaps I've found something to solidify Mountain level Pica and Zoro!
So first I am going to work out the size of the Flower Hill to find its dimensions before using PE (and 2 time-frames, and yes... the highest end is totally going to be 1 second :3) to find the energy required for Pica to have applied such a feat.
Here is where the feat starts. It ends in the very next panel. I guess using PE could work.
Referring back to my Zoro cutting Pica calculation, the height of the King's Plateau is roughly 2697 meters.
50 px = 2697m
91px = 4908.54m
Now to scale each layer... yay...?
The height of each layer is 1/4th of the total height of the flower hill, so each would be 1227.135m in height.
Each layer is circular, so a cylinder. the Formula is Pi*Radius^2*Height, so let's see the results.
124px (base) = 6688.56m (volume = 43116895080.4 m^3)
92px (2nd layer) = 5459.28m (Volume = 28724532101.7m^3)
60px (3rd layer) = 3560.4m (Volume = 12217428587.7m^3)
38px (top layer) = 2254.92m (Volume = 4900546355.73m^3)
Total volume = 88959402125.5
Mass = 88959402125.5*2700 = 2.4019039e+14 kg
Edit: Adding the weight of the Pica Statue.
I'm going to do this bit from scratch.
The Pica statue was initially part of the Flower Hill, and when the Pica Statue was first created, the Flower hill was visibly affected by this, as in it shrank. It must be added to the total value, seeing as how it was part of the initial creation of the flower hill.
Height of the Pica Statue = 5129.36618685m according to the Zoro calculation - roughly the same height as the Flower Hill itself, only a little taller. This is consistent with what is shown in the manga.
Now to scale the volume. For this, I'm going to use the real Pica's body as the scale, which works considering how the statue is a perfect replica, just massive. I will use the actual statue to scale as well, since we need the volume, and not just the area (This only applies for the torso). I'll also use median widths since scaling each part of an arm or leg would take too long.
Height = 5129.36618685m (655px)
Arm: (Green)
Median width = 151px (1182.49529313m)
height = 578px (4526.37271145m)
Volume, assuming a cylinder = 4970000000 m^3 *2 for the 2nd arm = 9940000000 m^3
Leg: (Orange)
Median Width = 80px (626.487572519m)
Height = 344.4445px (2697.37748341m)
Volume, again cylinder = 1662000000m^3 for 2 legs.
Torso: (Yellow - Also using 2nd image) Assuming it's a rectangular prism
Width (Image 1 only) = 326.524118558px
Height (Image 1) = 221px (1730.67191908m)
Height (Image 2) = 1730.67191908m (87.8px)
Thickness (Image 2) = 76.48px (1507.53752346m)
Volume, assuming a rectangular prism = 6670000000m^3
Head w/ neck: (Blue)
Height = 103px (806.602749618m)
Width = 84.012px (657.905171857m)
Volume, assuming a cylinder = 274000000m^3
Total Volume of Pica Statue = 274000000 + 6670000000 + 1662000000 + 9940000000 = 18546000000m^3
Now to include this volume w/ the original, so
18546000000 + 88959402125.5 = 107505402126m^3
Mass = 107505402126m^3*2700(kg/m^3) = 2.9026459e+14 -> not much different from before
PE (High end) = 2.9026459e+14*9.8*4908.54 = 1.3962798e+19 Joules, or 3.3372 Gigatons - Small Island level+ Assuming he did this in 1 second
Mid-end (10 Seconds) = 333.72 Megatons - Mountain level
Low-end (30 Seconds) = 111.24 Megatons - Mountain level (Accepted Value)