Designing a jumper

[BackforGood]

Can anyone tell me what proportion of the wool for a jumper, you would use for the front of the jumper ?

I'm going to knit a jumper for someone who wants a particular design on the front it's pretty geometric, so I think I can work out how many stitches of each colour to knit, but I'm not sure what to buy.

So, would it be reasonable to 'guesstimate that the 'back' is about 1/3 of the total wool, the front is about 1/3 and the 2 sleeves together make up another 1/3, or would sleeves be likely to be more than that (neckband would be needed from the sleeves and back colour too).

Or am I completely loopy to even attempt this ?