Extrapolating data within a table

[MacroIdiot]

Good morning,

I've been tasked with fixing up a spreadsheet to cut down a lot of time. I've found a UDF that can interpolate my data easily ( http://www.dailydoseofexcel.com/arch...ookup-between/ ).

My table has several columns and all but one are filled out to the extent of the X data, where as one column is cut off at about half way through. I'd like to extrapolate the incomplete data based on the last two points of information to the range of the X data and then I can use the interpolate function to grab any data inbetween. I know I can do this using the trend() function but is there a way to do it with a macro so anyone else using the spreadsheet can plug in their table of information (which may be have more or less lines of data)?

I've attached an example table -- I'm looking to extrapolate the C values to the range of the X values based on the last two available C points of data. Is this possible/if so is there an existing macro that I could use?

Thanks in advance for your help and I apologise if this is considered a really stupid question... but I learned a tiny bit of VB years ago and am simply not very familar with all of the Excel VB functions.