Simple, the yourself, that combined with openness, honesty and transparency. I personally think that a calculator (and whatever device your reading this on will have access to a calculator in one form or another), a calendar, a notebook and a pen.
Those are the simplest tools to use, but then there’s maths anxiety to deal with. DW suffers from maths anxiety, anything to do with numbers and she freezes. I’m starting to see math anxiety with the children as well. There must he something wrong with the way it’s taught.
I have finger trouble more than maths anxiety, my stumpy little fingers have a tendency to do transposition or key in totally random numbers. I learnt a long time ago to check and recheck my calculations. Maybe that’s the difference between the maths anxious and non-maths anxious. The non-maths anxious know that mistakes are made so check their workings. DW’s problem with maths initially cam from not showing her workings for the calculation, she’d do a sum in her head and not show the calcs, so her teacher got ratty.
Perhaps that’s happening to the DDs’, they not being taught how to learn how to do the calculations and not learning that you are allowed to make mistakes.
Like l say one of the reasons I’m not overly fond of personal finance tools on desktops, phones etc, is that a simple number transposition can wreak havoc in your personal finances, there is s big difference between 1913 and 1193, £800 if this was the case, thinking there was an extra £800 in the bank would cause us real problems.