Where are you using the 'compare' portion of your db? You can probably set it on the "on close" or "after update" on the subform to compare the data from that form to the data in the main form.
That's an idea... but I'm not sure which event I need. The subform changes because I change its recordsource based on a selected date in the main form. (The compare happens within the same function as that.) What kind of events does that trigger for the subform? I tried OnCurrent but that's still too early, the total isn't calculated yet.
How are you calculating the total from the Subform? I did something similar to this (kind of like an order screen, the subform was the line item detail, and the main form had the totals). From what I remember I put code in the main form and the subform. The main form used a Recordsetclone of the subform to do the calculation and I believe I would have done it in the OnCurrent event. Any more details?