Calc In Libreoffice: Querying Irregular Numerical Incrementation

[gl556tr]

In Calc, I have created a list of all my recordings on DVDs. As I will have more than 99 Titles, I have set the DVD № format in column Cx to 000. I need to show the **remaining time**, so that I can easily find the DVDs having space for more recordings. Each side has 6 hours of recording.
With the recording titles listed in column Ax, the DVD № are in column Cx, starting with C(1) = "001". The number of C-cells containing "001" varies w.r.t. to number of recordings on this first DVD. Similarly, for all subsequent C-cells.
Therein lies my problem in calculating the Remaining Time for each DVD. I currently have the following entries for Cx:
C(2)=001; C(3)=001; C(4)=001; C(5)=002; C(6)=002; C(7)=003;
C(8)=003; C(9)=003; C(10)=003; C(11)=003; C(12)=004.
I started writing a nested IF-statement, but soon realised this would be enormous and prone to error:
=IF(C2=C3;IF(C3=C4;IF(C4=C5;D4=0;D4=C4);D3=C3);D2=C2)
I really have run out of steam, and would appreciate any assistance offered. Surely, there must be somebody having had a similar task. Is there a means of incrementing C(X) until its content changes - i.e. "001" until "002"? Thank you.

[bibblebub]

I haven't used them myself but would the SUMIF() / SUMIFS() functions do what you want?