Finding the nth occurrence after a specified position

Hello

I am trying to find the very first occurrence of a tag "</td>" in a string after a specified position. I have attached the example spreadsheet.

The first occurrence of the tag "</td>" is at position 40 as in the column C all the occurrences are calculated for this tag.

However, I want to know what is the very first occurrence of the same tag "</td>" starting at position 100.

I have tried to write this formula in cell M4 but it throws up an error unless I say the 6th occurrence instead of 1st as in cell M8 (in the actual larger spreadsheet that I am working on, I wouldn't know if its the 6th occurrence that comes after the 100th character).

Does anyone know how to make the formula in M4 work?

Thanks
Regards