Land Drainage pipe calculation

[SortedPlumbing]

I am plumber who is assking you gardeners for advice here!

Installing land drains in my back garden as it is clay soil.
i have removed all my top soil and just about ready to dig trenches for land drains. The garden is 7m x 8m and slopes towards my house( so unable to dig soakaway) Looking for advice :

How to calculate what mm of pipe i need?

How far apart do i need to place the drains?

Only place that i think i can discard the water into would be my rain water underground drainage (which i know isnt ideal)
Does anyone know if there are good filters that could make this possible and prevent blockages?

 

[Thermo]

100mm pipe is fine. You can wrap it in geotextile fabric, but we dont when we do it, as it can block up over time. Its not a big garden. The surface water is gong to follow the slope down, so a pipe across the middle and at the botom will easily do th job.