Building ATX PSU - what current loads on the +3.3V, +5V and +12V rails?

[Elberoth]

I'm about to build an full linear ATX PSU for my next server/end point build. 

 

It will use a MoBo with an integrated low power CPU (10W TDP), that draws up to 3A @12V (3x12 = 36VA), depending on CPU load. It has the ATX power input. 

 

My question is this: how many watts to assign to each ATX power rail (+3.3V, +5V and +12V) ? 

 

Is the power draw evenly distributed between the rails, or is it say 80% on the +12V rail, 15%  on the +5V rail and 5% on the +3.3V rail?

 

My initial idea was to use:

 

+12V: 30 or 40VA power transformer + 3A voltage regulator 

+5V: 15VA or 20VA power transformer + 3A voltage regulator 

+3.3V: 15VA or 20VA power transformer + 3A voltage regulator 

 

The SSD drive and PCIe USB card will use their own transforemers and power supplies, so they will not increase the power draw of the MoBo. 

 

The PSUs/voltage rehulators that I plan to use are those: https://www.mpaudio.net/product-page/msw-hpuln-ps-single-rail-3a

 

At the moment, the MoBo is powered by 50VA transformer / 3A voltage regulator + PicoPSU which is more than enough (doesn't get hot or anything).