Have cholesterol levels tested. total: 4.6 mmol/l hdl:1.3 ldl:2.9 tryglycerides: 1.0 if i used formula should total value be ldl+hdl+ 20% triglycerides = 4.4 (not 4,6?) could this be due to difference between measuring and calculating or otherwise?

Lab tests are indicators, not diagnosticians. All lab tests shouldn't be interpreted by the physician who thought it prudent to order the tests. They should be interpreted in light of your physical status, meds, health history, and the guidance that they can give your practitioner. A 10 minute computer search its no substitute for a 600,000 minute medical education and aa lifetime of experience.