Variable-rate application gives each part of a field only the fertiliser it needs, instead of one blanket rate everywhere. The saving comes from how uneven your soil and yield already are — visibly patchy fields or mixed soil types see more benefit than flat, uniform ground. 15% is a reasonable starting point for a typical mixed arable field. Move it down if your fields already look even; up if parts of a field consistently under- or over-perform.

This is yield that would otherwise be lost to a problem — disease, pest pressure, a nutrient shortfall, waterlogging — that isn't noticed from the ground until some damage is already done. The earlier it's spotted, the more of that loss is avoidable. 5% assumes a fairly typical, moderate-risk season. Raise it if your fields have a history of problems being caught late, or if you don't get round to walking them often; lower it if you already scout closely.

Each survey is a chance to catch a developing problem at a key growth stage — establishment, tillering, stem extension, flowering and pre-harvest are the usual windows for a wheat or barley rotation. 5 a year covers those stages without over-flying. Fewer surveys leave gaps where something could develop unseen between visits; more gives tighter monitoring, at the cost of more survey credits.