I don't think that's always right. The recent deal for Indian Meals was so attractive because they were both half price and 20% off the original price, meaning that they ended costing less than a third (30%) of the original.
I bought a lot, but sadly most are gone now.

I think it's always worked that way. When an item is simply reduced in price, as here, the PYO discount is 20% off the current (reduced) price.

However if the product is on a multi-buy offer; BOGOF, 3-for-2, two-for-£5 etc., then the PYO discount is 20% of the full pre-offer price.

For example, a while ago 8-can packs of Pepsi Max were £3.50 BOGOF, and with PYO Offers you could buy two packs for £3.50 - (20%×£7) = £2.10.

What a lazy s*d I am - could have done that myself.
thanks accvio

Hi
Have seen several offers for waitrose PYOO on the forum.
How often can you change your chosen reductions?
Thanks

Good find. I thought this was going to be the first deal I posted, but you beat me to it!

In the shop I noticed that the Pick Your Own Offer option wasn't presented on the half price 750g, but was on the price-matched 500g packet. I therefore wondered if the PYOO would still work on the 750g, but it did.

Unfortunately the 20% came off the reduced price, not the original price, a glitch that Waitrose sometimes seem to have.

Nevertheless, a great deal, and I now have ten packets waiting to be eaten.

OTOH, I can't really tell the difference between these and the ASDA own brands which were three 500g packets for £3 recently.