One way to implement this is to use accruals accounting (which is typically built on double-entry accounting).

That is, enter the expenses when they are incurred rather than when they are spent. You’d also enter the income when earned rather than when received.

Then the balance of the account would be your ‘safe to spend’ amount.

Method 1: use your bank accounts

If you use just one bank account, a simple way to implement this is to create a Safe-to-Spend (Asset) account that we'll call "Available", plus all the Goal (Asset) accounts. The bank account is then a (virtual) aggregation of these asset accounts. You can do this in MYOB if you arrange the Safe-to-Spend and Goal accounts as sub-accounts of a summary account that we'll just call Bank here.

Setting goals and paying for things reduces your Available amount. So when you set goals or pay for things, you debit Goal (put money in) or Expense, and credit Available (take money out). It's possible that Available goes negative, in which case you know that you don't have any money that is safe to spend.

Assuming you are conscientious about entering your transactions, the summary "Bank" account always reflects the true total of your actual bank account.

Since transactions are all handled in the Available account, bank reconciliation will always be done with that account. You will need to manually reconcile movements between Available and Goal as that will not show up in your bank accounts.

You could open multiple bank accounts called "Goal 1", "Goal 2", ..., and "Available", in which case you won't need the summary "Bank" account. But you will have multiple statements to reconcile.

Method 2 use tracking accounts

You might have multiple bank accounts that aren't structured according to your Goals. For example, you might have a credit card account, a savings account and an investment account. The balances do not reflect expected mortgage payments or that nice car you've got your eye on.

In that case, you can create 'paired' tracking accounts. You'll need to be more careful because these accounts will not have bank statements for reconciliation.

Practically-speaking, it doesn't really matter where you put the tracking accounts (Asset, Liability, Equity, etc). My uni background was computer science and not accounting, so if you need to have these in 'proper' categories, please consult an accountant. I'll use Equity accounts here since that seems appropriate for the exercise of apportioning your equity.

Create Equity accounts Allocated, Available, Goal 1, Goal 2, etc.

When you earn income, you make 2 sets of double entries:

1. Income account // Asset (bank) account - this is the real bank transaction
2. Allocated account // Available account - this is your tracking transaction

When you create financial goals, you make 1 set of double entries:

1. Available account // Goal account

When you spend money, you make 2 sets of double entries:

1. Asset (bank) account // Expense account - this is the real bank transaction
2. Goal account // Available account - this is your tracking transaction (make sure this goes in the opposite direction compared to the 'creating goals' double entry).

Your Allocated account will keep growing. It tells you how much you have allocated over time.

Your Available account will contain your Safe-to-Spend amount.

Your Goal accounts will reflect the amounts set aside, which will decrease as that money gets spent.

Disclaimer: I am not an accountant and this answer is not financial advice. Please seek appropriate financial advice.