You cannot say : "it is mostly terrestrial, so I'll make a mostly terrestrial setup". Even if a newt only spends 3 months a year in the water, when aquatic, it's fully aquatic and its needs
For example, your newt, if alone, (about 18 cm or 7 inches as an adult) will need at least :
- 0,1 square meter of land, with a rather dry part
- 0,1 square meter of water with a depth of 18 cm
- an easy access transition zone between the two.
(multiply by the number of newts you have)
All that being said, you can choose to have a paludarium 80% land / 20% water, or 80% water / 20% land, or anything else if you want.
With a 80% land / 20% water solution, the 20% water should be at least 0,1 square meter for 1 newt.