Lisp that adds values and (if possible creates field/automatic update)

Does any one know if it would be possible to create a lisp that would add selected text (200f + 400f = 600f) its always a lowercase f and be updateable? so if i changed the value 200f to 400f it would change the output to 800f? Ive been trying with fields and havent gotten it to work succesfully if you know how to do it with fields that would be great too.

Can you post an example drawing? What you are asking seems rather simple, but I'm not sure I fully understand. To many factors to just guess on.

If the "f" wasn't there, Fields would work perfectly (barring I understand what's wanted. You could use object content fields and combine them in a formula field. See below:

attached is a portion of a building, the text in the clouds is whats added and the attribute that says total here is where i want the total in the end, being in mtext would be fine too, I cant get rid of the f

add cfm.dwg

If the "f" wasn't there, Fields would work perfectly (barring I understand what's wanted. You could use object content fields and combine them in a formula field. See below:

 

[ATTACH]15406[/ATTACH]

how do i combine them?

EDIT: figured it out for just numbers but cant seem to get it to work with the f

I solved my problem, i made a block for my cfm's and had the f as a piece of the block and the # for cfm as an attribute that is right aligned to the f, gonna take some time to replace for existing drawings but will save time in the future when i use it from the start. I attached my block in case anyone else has a similar problem and my explanation isnt enough

cfm.dwg