I've read enough to know that I'm likely to get the "don't do that" response to my question.
The thing is, it needs to be done.

I have a field on a form which contains a weight of a certain substance. Based on the weight of this substance, I need to calculate a target weight for a second substance based on a predefined ratio (55-45). The weight of the second substance is then measured and compared against the target weight to see if the compound passes or fails.

So there are three fields (controls, I guess): A (weight of substance A), B (target weight of A+B), and C (actual weight of A+B).
Is there a way to calculate B based on the data in A? I was thinking some sort of on exit property, but a little guidance before I delve in to that would be greatly appreciated.

are you actuality filling a table with values and data or just using the form as a fancy calculator?
are the ratios always the same?

Thanks for your response.

The table will contain data to be retrieved using queries, including date/time, batch number, and so on.
I will admit that this particular field is akin to a fancy calculator, but there are other values that need to be treated in this way (i.e. a database). This calculated value must be displayed to the end user, since it gives them a target weight. A pass/fail determination is then made based on the total weight and how close the actual ratio of the measured substances is within a set tolerance.

The ratio is always the same, yes. The compound is 55% substance A and 45% substance B.