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Subtotal and tax calculations in Magento 1

I have a Magento v1 store that I’m in the process of migrating to Shopify.

The existing orders from Magento have been migrated to Shopify using cart2cart.

Testing the orders in the new platform we’ve noted some oddities where orders show as if the customer has overpaid.

One example of these orders is screenshot here:

Screenshot showing order subtotal as 145.82, shipping as 0.00, discount of -17.50, tax of 26.25 and a grand total of 157.50

The original list price of the item sold here is indeed £175.00 – so the discount of £17.50 is correct.
However, based on this Magento basket summary of the order totals I can’t work out the order total to the same total it shows.

When this order has migrated to Shopify it has a difference owed to the customer of £2.92 – as they’ve calculated the order as being (shopify calculates the order total based on the values stored for items, shipping, tax etc rather than storing a total):

145.83 – 17.50 +26.25 = 154.58

Can anyone spot what’s going wrong?

I think the issue is the order in which the 10% discount is being applied when it comes to the tax calculation but I’m somewhat confused.