Deferment or discount rate
Prior to Sportelli (2006-2008) this varied through out the country and throughout central London. This case introduced the concept of a generic (nation-wide and property-wide) rate of 5%. Over night most premiums adjusted upward by 5%. Recently in Kelton Court, Edgbaston, due to a number of factors the Lands Tribunal determined a rate of 6%. Potentially your property is in an area where an adjustment to this rate is appropriate. The calculator only works off 5%. The higher rate, the lower the premium.