Well the wiki article about MB says its common usage is for either 10^6 or 2^20, and MiB is far less well known (but thanks for helping educate me :-) ). Choices are to:

1. Switch to MiB - which many non-technical people will find confusing but will be technically accurate and unambiguous.

2. Stick with MB - which will have widespread general understanding of roughly how big the cache will be, but will be slightly ambiguous.

I am going to stick with MB even though a little ambiguous - because it has the best user understanding and because choosing a cache size is at best a guess anyway, and because QT prunes the cache to 90% of what is requested anyway!!

If someone wants to put through a change following this one to change MB to MiB, I am fine with that.