How certain are we of the calculations?

We are 100% certain that we have implemented the official equations correctly (IPCC 2006). We have also implemented the official uncertainty calculations and provide the upper and lower bound of the official estimate. To our knowledge we are the only landuse focused greenhouse gas auditing tool to do this.

Why do we bother to report the upper and lower bounds on the calculations?

We are a farmer led company and as farmers we needed to know how certain any mitigation strategy we implement will really reduce the greenhouse gas emissions from our farms (i) because we fundamentally want to do the right thing and (ii) decisions we take this year influence the farm business for many years into the future. As farmers we are used to taking risks and appreciate that in order to make the right decision we need the best available information on which to base our decisions.

Why is there an upper and a lower estimate in the calculations?

It is not economically possible to measure all the greenhouse gas emissions from every farm so the emission factor approach was adopted by IPCC for landuse. Briefly this can be viewed as you collecting data on the price of the same type of biscuit/cookie by visiting 5 different shops - depending on the shop you visit the price may vary from 0.20 pence (cent,  €, $ or what ever currency is used in your country) to 0.50 pence (say you paid 20 pence , 32 pence, 35 pence, 40 pence and 50 pence). If I asked "what is the price of packet of biscuits/cookies ?" you might say on average 35 pence but the upper estimate is 50 and the lower 20. This is a very simple example - the actual methodology can be found in Vol 1 Chapter 3 IPCC 2006