Formulas that use the rules of a cell's data format... can it be done?

So I've exported my accounts for the year from my Squarespace store and I need to get the figures for sales without taxes. Sadly Squarespace doesn't allow for multiple levels of tax on products (we have 20% and 0%) so I need to input them manually for each product sold and get the total. Which is a bit of a pain but not the end of the world.

The problem is if I have a cell formatted as a currency (£) and I use a formula to subtract the cell formatted as a percentage (%) it's giving me an answer that's subtracting the figure as a whole number not as a percentage.

Here's screenshot showing what I mean. The sum should be...

£3.60 - 20% = £2.88

But it's doing...

£3.60 - 20 = £3.40

Even doing a sum that's -20% instead of the % cell doesn't work...

Is there a way around this apart from do a sum that's x0.8 on each line that needs it as there's 1000's of them and I need to pick out each which ones are each rate which will take far too long to do.

It seems to me to be a pretty straight forward thing to ask a spreadsheet to do but Numbers disagrees!

Any help would be much appreciated.