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
[TABLE="width: 500"]
<tbody>[TR]
[TD]1000 Current Read[/TD]
[TD]1000 Current Read[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]2000 Estimate[/TD]
[TD]800 Estimate[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]3000 Estimate[/TD]
[TD]600 Estimate[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]400 Actual Read[/TD]
[TD]400 Actual Read[/TD]
[TD]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[/TD]
[/TR]
</tbody>[/TABLE]



My Spreadsheet


[TABLE="width: 387"]
<!--StartFragment--> <colgroup><col width="129" span="3"> </colgroup><tbody>[TR]
[TD="class: xl43, width: 387, colspan: 3"]Regress Three Readings - divided by 4[/TD]
[/TR]
[TR]
[TD="class: xl39"] [/TD]
[TD="class: xl39"] [/TD]
[TD="class: xl39"] [/TD]
[/TR]
[TR]
[TD="class: xl36"] [/TD]
[TD="class: xl38"]Current Read[/TD]
[TD="class: xl38"]Updated Read[/TD]
[/TR]
[TR]
[TD="class: xl24"]Quarter 4[/TD]
[TD="class: xl30"]67898[/TD]
[TD="class: xl25"]67898[/TD]
[/TR]
[TR]
[TD="class: xl26"]Quarter 3[/TD]
[TD="class: xl31"]71000[/TD]
[TD="class: xl27"]67365[/TD]
[/TR]
[TR]
[TD="class: xl26"]Quarter 2[/TD]
[TD="class: xl31"]70000[/TD]
[TD="class: xl27"]66832[/TD]
[/TR]
[TR]
[TD="class: xl26"]Quarter 1[/TD]
[TD="class: xl31"]69000[/TD]
[TD="class: xl27"]66298[/TD]
[/TR]
[TR]
[TD="class: xl26"]Quarter 4[/TD]
[TD="class: xl32"]65765[/TD]
[TD="class: xl29"] [/TD]
[/TR]
[TR]
[TD="class: xl41, width: 258, colspan: 2"]Difference between Current and in line reading[/TD]
[TD="class: xl30"]2133[/TD]
[/TR]
[TR]
[TD="class: xl28"]Increment [/TD]
[TD="class: xl33"] [/TD]
[TD="class: xl34"]533[/TD]
[/TR]
[TR]
[TD="class: xl37"] [/TD]
[TD="class: xl37"] [/TD]
[TD="class: xl37"] [/TD]
[/TR]
[TR]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[/TR]
[TR]
[TD="class: xl35"] [/TD]
[TD="class: xl35"] [/TD]
[TD="class: xl35"] [/TD]
[/TR]
[TR]
[TD="class: xl43, colspan: 3"]Regress Two Readings - divided by 3[/TD]
[/TR]
[TR]
[TD="class: xl39"] [/TD]
[TD="class: xl39"] [/TD]
[TD="class: xl39"] [/TD]
[/TR]
[TR]
[TD="class: xl36"] [/TD]
[TD="class: xl38"]Current Read[/TD]
[TD="class: xl38"]Updated Read[/TD]
[/TR]
[TR]
[TD="class: xl24"]Quarter 3[/TD]
[TD="class: xl30"]25000[/TD]
[TD="class: xl25"]25000[/TD]
[/TR]
[TR]
[TD="class: xl26"]Quarter 2[/TD]
[TD="class: xl31"]34567[/TD]
[TD="class: xl27"]23782[/TD]
[/TR]
[TR]
[TD="class: xl26"]Quarter 1[/TD]
[TD="class: xl31"]31234[/TD]
[TD="class: xl27"]22563[/TD]
[/TR]
[TR]
[TD="class: xl26"]Quarter 4[/TD]
[TD="class: xl32"]21345[/TD]
[TD="class: xl29"] [/TD]
[/TR]
[TR]
[TD="class: xl46, width: 258, colspan: 2"]Difference between Current and in line reading[/TD]
[TD="class: xl40"]3655[/TD]
[/TR]
[TR]
[TD="class: xl28"]Increment [/TD]
[TD="class: xl33"] [/TD]
[TD="class: xl34"]1218[/TD]
[/TR]
[TR]
[TD="class: xl37"] [/TD]
[TD="class: xl37"] [/TD]
[TD="class: xl37"] [/TD]
[/TR]
[TR]
[TD][/TD]
[TD][/TD]
[TD][/TD]
[/TR]
[TR]
[TD="class: xl35"] [/TD]
[TD="class: xl35"] [/TD]
[TD="class: xl35"] [/TD]
[/TR]
[TR]
[TD="class: xl43, colspan: 3"]Regress One Reading - divided by 2[/TD]
[/TR]
[TR]
[TD="class: xl39"] [/TD]
[TD="class: xl39"] [/TD]
[TD="class: xl39"] [/TD]
[/TR]
[TR]
[TD="class: xl36"] [/TD]
[TD="class: xl38"]Current Read[/TD]
[TD="class: xl38"]Updated Read[/TD]
[/TR]
[TR]
[TD="class: xl24"]Quarter 1[/TD]
[TD="class: xl30"]3456[/TD]
[TD="class: xl25"]3456[/TD]
[/TR]
[TR]
[TD="class: xl26"]Quarter 4[/TD]
[TD="class: xl31"]4567[/TD]
[TD="class: xl27"]2795[/TD]
[/TR]
[TR]
[TD="class: xl26"]Quarter 3[/TD]
[TD="class: xl32"]2134[/TD]
[TD="class: xl29"] [/TD]
[/TR]
[TR]
[TD="class: xl46, width: 258, colspan: 2"]Difference between Current and in line reading[/TD]
[TD="class: xl40"]1322[/TD]
[/TR]
[TR]
[TD="class: xl28"]Increment [/TD]
[TD="class: xl33"] [/TD]
[TD="class: xl34"]661[/TD]
[/TR]
<!--EndFragment--></tbody>[/TABLE]