Came across this link at bikeradar earlier regarding tyre pressures for anything up to fatbike size...
https://www.youtube.com/watch?v=Clk_LLBYFzA
Chap has created a spreadsheet where you plug in your figures and it suggest pressures. Direct link to dropbox xls file (which I discovered works fine in Google Sheets)...
https://www.dropbox.com/s/v149cjesl7ht35g/FFT_tyre_pressure_calculator.xls
Suggested I should be running my 38c Marathon Cross at ~38/60PSI, but given how that front is less than official minimum on tyre, use 50PSI.
Came back from my extended route home and decided to boost pressures from ~55/61PSI I've been running the last few days to ~63/71PSI, as things felt a bit draggy. However, in fairness, my legs were a bit tired straight after work and it was a bit windy (sadly headwinds and crosswinds).