What % readings should I be aiming at to maintain good lawn watering levels 
( Uk based)
For most UK lawns, aim for 35–50% soil moisture. That’s a solid range for the cool-season grasses we typically have here (fescue, ryegrass, etc.).
A few things to keep in mind:
Soil type matters a lot
Sandy/chalky soil (common in the south/east): dries out fast, aim for 25–35%
Clay soil (heavier, common in many UK gardens): holds water longer, 40–55% is fine
Loam (the lucky ones): 35–45% is ideal
UK-specific tip
Honestly, most established UK lawns don’t need much supplemental watering outside of proper dry spells. Rainfall from autumn to spring is usually plenty. If you’re using sensors, set your “stop watering” threshold around 40–45% and only water when it actually drops below that.
Don’t over-water
If you’re consistently above 55–60%, you’re risking fungal issues and shallow roots. Better to water deeply and infrequently than little and often.