There are people that estimate based on a base wage and then they work overtime so jump online and update a thousand extra. Some people update once a month as their circumstances change. I would generally suggest looking at best case scenario in terms of wages and work it out from their (so best wage you could expect) and then get a too up at tax time.

The only other option is to not claim ftb until tax time. But honestly, given previous posts regarding your financial situation I would assume that's an idea you can't afford to entertain at this stage.

I will suggest to try and always estimate higher than you think you will get. Of you estimate low and then earn more you may get a debt a t tax time. Or when you update your estimate you will have a big reduction in payments.