Finding the relative position of the last X

Hi,

One of process lines needs cleaning every 4,24 and 500 times an item goes through it.
Excel Query 3.xlsx

So starting at the first point roll one it assumed that all counters have been reset.

At the end of job 4 the operators are to stop and clean the line. Once complete they are to put an X in that column and they can carry on to item 5.

What I want to do is get a visual representation of number of jobs to clean.

Talking now about the attached spreadhseet.

In Column F the last X is in cell F50, This relates to A50 as it is the 48th item produced. We have then gone on to produce items upto 53. So we have produce 5 items without a clean so we are 1 over due and this number 5 in this case is number that is important to get automatically.

So my basic understanding of how the formula would work is look up the last cell with X in For Column F this F50, G50 and H2. I then want to know what value is in column A at this point and find out what the highest number(=MaxA:A) is in Column a and subtract the two to give 5,5,53.