detect lower number in list and makes calculations

[bumfart66]

I've written a basic spreadsheet to calculate new electric bills when readings are estimated and then a lower reading is given by a customer , for example 65765 is an actual reading then 69000, 70000, and 71000 are estimated, then the customer gives a reading of 67898. If this happens we have to divide the difference by 4 and add this to the estimated readings to recalculate bills, the system won't let you enter a lower reading when a bill has been generated. We only have to regress bills when there are either 1,2 or 3 bills to regress. My spreadsheet is dumb because i have to write it for 3 scenarios, shown below


What i would like is a spreadsheet where you enter the readings in column A and it calculates when a lower number is added and then makes the calculation as below, for example

1000 Current Read	1000 Current Read
2000 Estimate	800 Estimate
3000 Estimate	600 Estimate
400 Actual Read	400 Actual Read	excel detects lower than 1000, subtracts lower actual read from current read, divides by 3 and adds the difference to two estimated reads leaving the current read as 100

<tbody>
</tbody>

My Spreadsheet

Regress Three Readings - divided by 4
	Current Read	Updated Read
Quarter 4	67898	67898
Quarter 3	71000	67365
Quarter 2	70000	66832
Quarter 1	69000	66298
Quarter 4	65765
Difference between Current and in line reading	2133
Increment		533
Regress Two Readings - divided by 3
	Current Read	Updated Read
Quarter 3	25000	25000
Quarter 2	34567	23782
Quarter 1	31234	22563
Quarter 4	21345
Difference between Current and in line reading	3655
Increment		1218
Regress One Reading - divided by 2
	Current Read	Updated Read
Quarter 1	3456	3456
Quarter 4	4567	2795
Quarter 3	2134
Difference between Current and in line reading	1322
Increment		661

<!--StartFragment--> <colgroup><col width="129" span="3"> </colgroup><tbody>
<!--EndFragment--></tbody>