ss wrote

Not quite DIY:

Corse it is instead of getting the accountant or book keeper to do it.

Not having used mark up and margin for years I am now confused as to what
my profit is in % terms.
example:
cost price £1.89
sell price £2.89
Profit is £1.00

But should I log this as 53% profit based on cost price

Yes.

OR 34% profit based on selling price.

Nope.

At the moment I am thinking it should be 53% based on cost.

Yes.