simple formula problems

[Brad Hajek]

I have reviewed the 4 links in the FAQ in regards to using variables within
scheduled transaction formulas, but have yet to get even a simple formula
to evaluate correctly. I'm certain I'm just misunderstanding the
documentation.

Here is one of the formulas I'm working with for a car loan:

checking account debit: 338.10
expense account credit: interest
liability account credit: 338.10-interest

The scheduled transaction box always prompts me as expected for the
interest variable. Based on my reading of the FAQ links, I've configured
the subtraction formula with parenthesis around it and a colon-equals in
front of it without any success. I've also tried several different names
for the variable in case 'interest' is already taken for internal purposes.

However, no matter what I've tried it comes out unbalanced like this:

checking account debit: 338.10
expense account credit: interest
liability account credit: 338.10
imbalance-USD account debit: interest

Please tell me what I'm doing wrong. Thanks.

Brad