I havent written anything, besides putting my Current Weight, Starting Weight and Goal Weight in my postbit.

I am no good with coding at all, and have no idea what to put in their for the thigns, even after looking at your article:(

Well, you are going to have to try something in order to see if it will work. You know the equations, right? I can't write any of this for you, you need to write them.

Rewrite these with the correct fields:

$diff = $starting - $current
$diff = $post[fieldx] - $post[fieldy] ???

Rewrite that using the correct field numbers. Now put it into a condition like the article says:
and then spit out the variable $diff

Then, if you are going to say "It doesn't work!" you need to post exactly what you wrote and tell us what the result is.

I entered

<if condition="$post[field13] = $post[field10]-$post[field11]"></if>

And nothing else shows up in psotbit at all

Unless you create a plugin you will have to edit 'class_postbit.php' and insert the following code inside the function 'process_registered_user'
$post['field13'] = $post['field10']-$post['field11'];

Does that work?

If you need it to be saved somewhere, then you will have to write a plugin to save it to the database. If you just need it spit out in the postbit, then just try what I wrote above.