Previous Voucher Value

Hello Experts
I have User Defined Field in Sales Voucher,
Entering numbers, for example
15-05-12 1st voucher = 25
16-05-12 2nd voucher = 39
17-05-12 3rd voucher = 1000
01-05-12 4th voucher = 45
Now i want to get value of previous/last voucher (4th voucher = 45+1) in next 5th voucher, means
02-05-12 5th voucher = 46

Using below i am getting last value + 1 that is 1000+1, i need 45+1
[System: UDF]
storeBillRangefrom: String: 1029

[Field : Billrangefrom]
Use : Name Field
Storage: storeBillrangefrom
Set As : $CurrentBillRangefrom
Width : 10
[Collection : HPCollection]
Type : Voucher
Filter : IsSales

[System : Formula]
LastBillRangefrom : $$CollectionField:$storeBillRangeto@LastItem:HPCollection
CurrentBillRangefrom : $$Number@LastBillRangefrom + 1

 

Thanks sir, its working.