When you have pump it does not matter much if you split from 1 inch into 1/2 inch since the pump has enough force to push up the water pressure.

When you have no pump logic tells you that the pressure it has is the height of the tank location to its outlets. It is all about gravity and the pressure is usually low due to the limited height. In such case you got to consider the right size of pipe with below consideration:

1. Water that run flat will be slow as such you need bigger pipe. This is from Tank to each individual bathroom before going down.

2. water that runs down has gravity assist therefore need not have to use big pipe, it doesn't help much or at all.

3. The outlets are 1/2 inch, having bigger than half inch does not help the volume flow for pipe coming down with gravity.

In conclusion the proper sizing is:

1. From tank running flat = 1 inch. 1 inch pipe can support 4 nos of 1/2 inch pipes without suffering lost of pressure.
2. From roof to bathroom = 1/2 inch

Note: Area of circle = PI R square.
1 inch pipe area = (22/7*1/2)x(22/7*1/2)=2.47sq"
1/2 inch pipe area =0.617sq"
2.47/0.617=4