VBA macro to add flag to column, with predefined parameters

[NickWels]

Hi

I need help with a VBA macro. a few steps and logic.
File already open via import from text

4 step message box:
1. Enter value
2. Enter symbol
3. Choose column for flagging
4. Choose column to match

The value entered (usually around 1000-2500) will be used to add the symbol in the chosen column for every multiple instance, but to the nearest populated row in another chosen column
i.e 1000 entered = 1000,2000,3000, and so on to end of doc but would be something like 1005, 2008, 2999, 4001 when matched against the other column

I've created a 20k sample file. In sample file. Add flag to C, to the nearest * populated in A

Data will have a sequence column if this will be useful for anyone to use, feel free..

Many thanks for any help!