setting weekly Target

[hk2013]

i have a group of book seller and they have certain amount of books to sell by the end of year. We have already set monthly targets 

i want to get how many books they have to sell every week to get to get to the monthly target. Example:They have a target of 35% for jan how many books do they have to sell every week to reach that target. The weekly predict number should as more or less books are sold. if the sold less than their expected book than their next weeks number should be higher and if they sold more than expected than their next week number would be lower. 

data

Name	ID	Total Books	unsold books	sold books
Jon	J55	456	426	30
Adam	J79	386	357	29
Rob	J34	523	476	47
Tom	J91	350	332	18
Dustin	J28	450	425	25
Nancy	J44	343	324	19

Target: 

Month	Monthly_target
january	35%
february	50%
march	62%
april	71%
may	78%
june	84%
july	88%
august	92%
september	94%
october	96%
november	98%
december	100%

want:

Name	ID	Total Books	unsold books	sold books	weeks	pred_sold
Jon	J55	456	426	30	6-Jan	….
Jon	J55	456	426	30	13-Jan	….
Jon	J55	456	426	30	20-Jan	…
Jon	J55	456	426	30	27-Jan	…
Jon	J55	456	426	30	3-Feb
Jon	J55	456	426	30	10-Feb
Jon	J55	456	426	30	17-Feb
Jon	J55	456	426	30	24-Feb

[Reeza]

How is prep_sold calculated? What is the formula/logic to calculate the amount? I'm assuming this is cumulative as well, ie the 50% in February accounts for amounts sold in January as well? Sounds like you're trying to develop a 'how much should you sell' not an actual prediction of what would be sold? Otherwise if you miss a target for one month, wouldn't the 'more accurate' estimate be one that was lower than the previous value?

[hk2013]

yes i am trying to figure out how much i should see to meet the monthly targets

pred_sold= (((Total_book*0.35)- ((Total_book)-unsold books)))/(number of week to end of month)
pred_sold = (((456*.35) - ((456)-426)))/(3weeks) = 43.2
and next week when i recalculated it will (only number that changes is the unsold books)
(((456*.35) - ((456)-400)))/(2weeks) = 51.8