That was one of the reasons why I posted the "easy to make shaped tarp" thread but due to the great luck of interest I decided to delete it..
However here it is.(custom made for you...)
The idea is based on an irregular pentagon.
Depending on the starting size of your tarp and your own size (8x10//10x12) it can work better in one of two ways.
Lets call them Horizontal (pic N2) and Vertical (pic N3).
Horizontal is where the apex is in the middle of the vertical (short IE 290cm for you) side , vertical where it is in the middle of the horizontal (long IE 350cm ) side
This is the end result . The models are to scale (29x35 cm) the piece of rectangular paper is standing for a 180cm x 60 cm person.
The one on the left is the V type, the one on the right is the H type.
This is how is done :
Note that the lenghth of the top/bottom (H) or left/right (V) side is half of the starting size ( 185 for the 350 and 145 for the 290)
To get from the xehagonal figure to the Pentagonal one you fold in (and later cut) one of the two narrow sides (left or right, 145cm, in the V type.
The H type has a slightly higher apex (pole height) about 1 meter for this 350x290 version, about 90cm or so for the V type (sorry I could not be bothered working that out...)
What I suggest is that you start with a piece of paper like me , then either get hold of a a piece of painter's drop sheet of about that size (locally 260x360 cm, so get two and add the extra bit using tape) or mark your Hilleberg tarp with chalk, fold in and tape with Gaffa tape or the like the folded bits and see which shape if any works for you.
Before you actually cut your tarp, remember that you need to leave an extra few centimeters to the cut sides for the hems.
If this is not clear, please ask.
If you start with a square (10x10/12x12) then the top/bottom side is still half of the original, IE 5 and 6