Hi team, building a (hopefully) straightfoward Suitelet to allow us to accurately reconcile landed cost estimates vs actuals (FX involved and the bosses want a good indication of how close we are). Basic plan is to have a few header fields then a sublist populated from a saved search, and checkbox to select each transaction to be dealt with. Pretty confident with most of that, but I am hoping to have a header field that shows the 'balance' of the selected transactions (ie the variance). Is this best achieved via a fieldUpdated script for the sublist to add/subtract the value of the relevant line when the checkbox is ticked/unticked, or is there a better way to handle this?

Yep thats probably your best bet. If you are using the native Select All buttons that you get from sublist.addMarkAllButtons() I dont think it triggers a fieldChange event. Thats the only issue you may run into. Ive always just made my own mark all buttons though which fixes that problem.