Hi Toby! Carnation WA seems a very similar climate to Cambridge Uk, but with a lot more rain in winter which is good for gshp as it makes the ground more conductive. I recommend looking at the doc below - it takes a bit of getting your head around, but will predict minimum pipe length at a certain spacing and depth, with borehole/linear/slinky all covered. Ground conductivity seems to be the most crucial aspect of it all.
https://www.gshp.org.uk/pdf/MIS_3005...ing_tables.pdf
All the temperatures here are being logged, I like graphs, I’ll add one in later to show behaviour of the heat field over time.