How to accurately calculate discount percentage in Shopify?

[Corina1]

Hello,

Does anyone know how to get the correct discount percentage in Shopify? 

What I get is a roundup percentage, irrespective of the figures.

So if I set a campaign with all prices to be -15% and if the initial price is 399 eur and the promo price is 339 eur, then the discount that appears is -16% instead of -15%.

To be crystal clear:

Initial Price = 399

Promo Price = 339

Discount should be = 1- 339/399 = 15.04% 

Instead of 15%, Shopify displays 16%.

Please help!

Thank you,

Corina